feat: Replace explicit priorities with stack-based priority modifiers

Simplifies effect priority management by using relative modifiers instead
of explicit numbers, making timeline reordering much more practical.

## New Priority System

Effects now use priority modifiers after EFFECT keyword:
- `+` increment priority by 1 (or start at 0 if first)
- `=` keep same priority as previous effect
- `-` decrement priority (or start at -1 if first, for background)

Old syntax:
```
EFFECT FlashEffect 0.0 0.5 0
EFFECT FadeEffect 0.1 0.3 1
EFFECT BgCube 0.2 3 -1
```

New syntax:
```
EFFECT - BgCube 0.2 3        # Priority -1 (background)
EFFECT + FlashEffect 0.0 0.5  # Priority 0
EFFECT + FadeEffect 0.1 0.3   # Priority 1
```

## Benefits

✓ Reordering effects no longer requires renumbering all priorities
✓ Priority relationships are explicit and relative
✓ File order matches render order (easier to understand)
✓ Same-priority effects clearly marked with `=`
✓ Background layers (-1) clearly marked with `-`
✓ Reduces errors when reorganizing timelines

## Implementation

- Updated seq_compiler to parse +/=/- modifiers
- Tracks current priority per sequence
- First effect sets base priority (0 or -1)
- Subsequent effects modify relative to previous
- Generated C++ code unchanged (still uses integer priorities)

## Changes to demo.seq

- All effects updated to use new syntax
- Effects reordered by priority within sequences
- Priority gaps removed (were likely unintentional)
- Comments updated to reflect new system

## Documentation

- Updated SEQUENCE.md with new syntax
- Added examples showing +, =, - usage
- Explained priority calculation rules

This makes timeline authoring significantly more maintainable, especially
when experimenting with different effect orderings during development.

4 files changed, 147 insertions, 87 deletions

diff --git a/assets/demo.seq b/assets/demo.seq
index 35935a7..44df234 100644
--- a/assets/demo.seq
+++ b/assets/demo.seq
@@ -8,10 +8,10 @@
 #
 # QUICK REFERENCE:
 #   SEQUENCE <start> <priority> [optional_end]
-#     EFFECT <ClassName> <start> <end> <priority>
+#     EFFECT <+|=|-> <ClassName> <start> <end>
 #
+#   Priority modifiers: + (increment), = (same), - (decrement/background)
 #   Time notation: 0b (beats), 0.0 (seconds)
-#   Priority: Higher numbers render later (on top)
 #
 # VALIDATION & VISUALIZATION:
 #   ./build/seq_compiler assets/demo.seq                    # Validate only
@@ -19,86 +19,88 @@
 #
 # ============================================================================
 
+# BPM 120
+
 SEQUENCE 0b 0
-  EFFECT FlashEffect 0.0 1. 0
-  EFFECT FadeEffect 0.1 1. 1         # Add fade
-  EFFECT FlashCubeEffect .2 3 -1      # Background cube (priority -1 = behind everything)
-  EFFECT SolarizeEffect 0 4b 3           # Color inversion (last)
+  EFFECT - FlashCubeEffect .2 3       # Background cube (priority -1 = behind everything)
+  EFFECT + FlashEffect 0.0 1.         # Priority 0
+  EFFECT + FadeEffect 0.1 1.          # Priority 1
+  EFFECT + SolarizeEffect 0 4b        # Priority 2 (was 3, now contiguous)
 
 SEQUENCE 4b 0
-  EFFECT FlashEffect 0.0 0.2 4        # Add flash after solarize
-  EFFECT FlashCubeEffect 0.1 3. -1
+  EFFECT - FlashCubeEffect 0.1 3.     # Priority -1
+  EFFECT + FlashEffect 0.0 0.2        # Priority 0 (was 4, now contiguous)
 
 SEQUENCE 6b 1
-  EFFECT ParticlesEffect 0 4 1       # Particles layer
-  EFFECT GaussianBlurEffect 0 8 1       # Blur
+  EFFECT + ParticlesEffect 0 4        # Priority 0
+  EFFECT = GaussianBlurEffect 0 8     # Priority 0 (same layer)
 
 SEQUENCE 7b 0
-  EFFECT FadeEffect 0.1 1.0 5         # Add fade
-  EFFECT HeptagonEffect 0.0 .2 0        # Main geometric effect
+  EFFECT + HeptagonEffect 0.0 .2      # Priority 0
+  EFFECT + FadeEffect 0.1 1.0         # Priority 1 (was 5, now contiguous)
 
 # Post-processing chain (priority 10 = applied after scene rendering)
 # Effects are applied in priority order: lower numbers first
 SEQUENCE 8b 3
-  EFFECT ThemeModulationEffect 0 4 0    # Brightness modulation (first)
-  EFFECT HeptagonEffect 0.0 4.0 0        # Main geometric effect
-  EFFECT GaussianBlurEffect 0 8 1       # Blur
-  EFFECT ChromaAberrationEffect 0 6 2   # Color separation
-  EFFECT SolarizeEffect 0 10 3           # Color inversion (last)
+  EFFECT + ThemeModulationEffect 0 4     # Priority 0
+  EFFECT = HeptagonEffect 0.0 4.0        # Priority 0 (same layer)
+  EFFECT + GaussianBlurEffect 0 8        # Priority 1
+  EFFECT + ChromaAberrationEffect 0 6    # Priority 2
+  EFFECT + SolarizeEffect 0 10           # Priority 3
 
 SEQUENCE 12b 2
-  EFFECT FlashCubeEffect .2 3 -1      # Background cube (priority -1 = behind everything)
-  EFFECT HeptagonEffect 0 4 0
-  EFFECT ParticlesEffect 0 4 1       # Particles layer
+  EFFECT - FlashCubeEffect .2 3          # Priority -1 (background)
+  EFFECT + HeptagonEffect 0 4            # Priority 0
+  EFFECT + ParticlesEffect 0 4           # Priority 1
 
 SEQUENCE 15b 2
-  EFFECT FlashCubeEffect .2 3 -1      # Background cube (priority -1 = behind everything)
-  EFFECT FlashEffect 0.0 1 0
+  EFFECT - FlashCubeEffect .2 3          # Priority -1 (background)
+  EFFECT + FlashEffect 0.0 1             # Priority 0
 
 SEQUENCE 16b 10
-  EFFECT FlashCubeEffect .2 3 -1      # Background cube (priority -1 = behind everything)
-  EFFECT GaussianBlurEffect 0 8 1       # Blur
-  EFFECT FlashEffect 0.0 0.2 4        # Add flash after solarize
-  EFFECT FlashEffect 1b 0.2 4        # Add flash after solarize
+  EFFECT - FlashCubeEffect .2 3          # Priority -1 (background)
+  EFFECT + GaussianBlurEffect 0 8        # Priority 0
+  EFFECT + FlashEffect 0.0 0.2           # Priority 1
+  EFFECT = FlashEffect 1b 0.2            # Priority 1 (same layer)
 
 SEQUENCE 17b 2
-  EFFECT ThemeModulationEffect 0 4 0    # Brightness modulation (first)
-  EFFECT HeptagonEffect 0.2 2.0 1        # Main geometric effect
-  EFFECT ParticlesEffect 0 4 1       # Particles layer
-  EFFECT GaussianBlurEffect 0 8 3       # Blur
-  EFFECT Hybrid3DEffect 0 4 2        # 3D objects (priority 2 = foreground)
-  EFFECT ChromaAberrationEffect 0 6 4   # Color separation
+  EFFECT + ThemeModulationEffect 0 4     # Priority 0
+  EFFECT + HeptagonEffect 0.2 2.0        # Priority 1
+  EFFECT = ParticlesEffect 0 4           # Priority 1 (same layer)
+  EFFECT + Hybrid3DEffect 0 4            # Priority 2
+  EFFECT + GaussianBlurEffect 0 8        # Priority 3
+  EFFECT + ChromaAberrationEffect 0 6    # Priority 4
 
 SEQUENCE 24b 1
-  EFFECT ThemeModulationEffect 0 8 0    # Brightness modulation (first)
-  EFFECT HeptagonEffect 0.2 2.0 1        # Main geometric effect
-  EFFECT Hybrid3DEffect 0 20 2        # 3D objects (priority 2 = foreground)
-  EFFECT GaussianBlurEffect 0 8 3       # Blur
-  EFFECT ChromaAberrationEffect 0 10 4   # Color separation
-  EFFECT SolarizeEffect 0 10 5           # Color inversion (last)
+  EFFECT + ThemeModulationEffect 0 8     # Priority 0
+  EFFECT + HeptagonEffect 0.2 2.0        # Priority 1
+  EFFECT + Hybrid3DEffect 0 20           # Priority 2
+  EFFECT + GaussianBlurEffect 0 8        # Priority 3
+  EFFECT + ChromaAberrationEffect 0 10   # Priority 4
+  EFFECT + SolarizeEffect 0 10           # Priority 5
 
 SEQUENCE 32b 0
-  EFFECT ThemeModulationEffect 0 4 0    # Brightness modulation (first)
-  EFFECT HeptagonEffect 0 16 1        # Main geometric effect
-  EFFECT ChromaAberrationEffect 0 16 3   # Color separation
-  EFFECT GaussianBlurEffect 0 8 4       # Blur
+  EFFECT + ThemeModulationEffect 0 4     # Priority 0
+  EFFECT + HeptagonEffect 0 16           # Priority 1
+  EFFECT + ChromaAberrationEffect 0 16   # Priority 2
+  EFFECT + GaussianBlurEffect 0 8        # Priority 3
 
 SEQUENCE 48b 0
-  EFFECT ThemeModulationEffect 0 4 0    # Brightness modulation (first)
-  EFFECT HeptagonEffect 0.2 2.0 1        # Main geometric effect
-  EFFECT GaussianBlurEffect 0 8 3       # Blur
-  EFFECT SolarizeEffect 0 2 5           # Color inversion (last)
+  EFFECT + ThemeModulationEffect 0 4     # Priority 0
+  EFFECT + HeptagonEffect 0.2 2.0        # Priority 1
+  EFFECT + GaussianBlurEffect 0 8        # Priority 2
+  EFFECT + SolarizeEffect 0 2            # Priority 3
 
 SEQUENCE 56b 0
-  EFFECT ThemeModulationEffect 0 8 0    # Brightness modulation (first)
-  EFFECT HeptagonEffect 0.2 2.0 0        # Main geometric effect
-  EFFECT Hybrid3DEffect 0 4 1        # 3D objects (priority 2 = foreground)
-  EFFECT HeptagonEffect 0 16 2        # Main geometric effect
-  EFFECT ChromaAberrationEffect 0 16 3   # Color separation
-  EFFECT GaussianBlurEffect 0 8 4       # Blur
+  EFFECT + ThemeModulationEffect 0 8     # Priority 0
+  EFFECT = HeptagonEffect 0.2 2.0        # Priority 0 (same layer)
+  EFFECT + Hybrid3DEffect 0 4            # Priority 1
+  EFFECT + HeptagonEffect 0 16           # Priority 2
+  EFFECT + ChromaAberrationEffect 0 16   # Priority 3
+  EFFECT + GaussianBlurEffect 0 8        # Priority 4
 
 SEQUENCE 62b 0
-  EFFECT ThemeModulationEffect 0 3 0    # Brightness modulation (first)
-  EFFECT SolarizeEffect 0 3 5           # Color inversion (last)
+  EFFECT + ThemeModulationEffect 0 3     # Priority 0
+  EFFECT + SolarizeEffect 0 3            # Priority 1
 # Demo automatically exits at this time (supports beat notation)
 END_DEMO 65b
diff --git a/doc/SEQUENCE.md b/doc/SEQUENCE.md
index 05cdc93..614339e 100644
--- a/doc/SEQUENCE.md
+++ b/doc/SEQUENCE.md
@@ -79,17 +79,26 @@ SEQUENCE <global_start> <priority> [optional_end]
 
 ### EFFECT Declaration
 ```
-EFFECT <EffectClassName> <local_start> <local_end> <priority> [constructor_args...]
+EFFECT <+|=|-> <EffectClassName> <local_start> <local_end> [constructor_args...]
 ```
 
 **Parameters:**
+- Priority modifier (one of):
+  - `+`: Increment priority (or start at 0 if first effect)
+  - `=`: Keep same priority as previous effect
+  - `-`: Decrement priority (or start at -1 if first effect, for background layers)
 - `EffectClassName`: C++ class name (must be declared in `demo_effects.h`)
 - `local_start`: Start time relative to sequence start (beats or seconds)
 - `local_end`: End time relative to sequence start (beats or seconds)
-- `priority`: Render order within sequence (lower = drawn first/background)
-  - Negative priorities draw behind everything
 - `constructor_args`: Optional additional parameters (rarely used)
 
+**Priority System:**
+- Effects are assigned priorities based on their order in the file
+- First effect: `+` starts at 0, `-` starts at -1, `=` starts at 0
+- Subsequent effects: `+` increments, `=` keeps same, `-` decrements
+- Priorities determine render order (lower = drawn first/background)
+- Effects should be listed in desired priority order within each sequence
+
 ---
 
 ## Time Notation
@@ -223,36 +232,46 @@ White flash on strong beat hits.
 ### Basic Sequence
 ```
 SEQUENCE 0 0
-  EFFECT FlashEffect 0.0 0.5 1    # Starts immediately, runs 0.5s, priority 1
-  EFFECT HeptagonEffect 0.2 10 0  # Starts at 0.2s, runs until 10s, priority 0
+  EFFECT + FlashEffect 0.0 0.5     # Priority 0 (first with +)
+  EFFECT + HeptagonEffect 0.2 10   # Priority 1 (increment)
+```
+
+### Same Priority (Layered Effects)
+```
+SEQUENCE 0 0
+  EFFECT + Flash 0.0 0.5           # Priority 0
+  EFFECT = Fade 0.1 0.3            # Priority 0 (same as Flash)
+  EFFECT + Other 0.2 3             # Priority 1 (increment)
+```
+
+### Background Elements
+```
+SEQUENCE 0 0
+  EFFECT - FlashCube 0 10          # Priority -1 (background)
+  EFFECT = BgEffect 0 5            # Priority -1 (same layer)
+  EFFECT + MainEffect 0 10         # Priority 0 (foreground)
 ```
 
 ### Sequence with Explicit End Time
 ```
 SEQUENCE 8b 0 [5.0]
-  EFFECT ParticlesEffect 0 120 1  # Would run 120s, but sequence ends at 5s
+  EFFECT + Particles 0 120         # Would run 120s, but sequence ends at 5s
 ```
 
 ### Post-Processing Chain
 ```
 SEQUENCE 0 10
-  EFFECT GaussianBlurEffect 0 60 1      # Applied first
-  EFFECT ChromaAberrationEffect 0 60 2  # Applied second
-  EFFECT SolarizeEffect 0 60 3          # Applied last
-```
-
-### Background Element
-```
-SEQUENCE 0 0
-  EFFECT FlashCubeEffect 0 10 -1  # priority -1 = background layer
+  EFFECT + GaussianBlur 0 60       # Priority 0 (applied first)
+  EFFECT + ChromaAberration 0 60   # Priority 1 (applied second)
+  EFFECT + Solarize 0 60           # Priority 2 (applied last)
 ```
 
 ### Music-Synchronized Effects
 ```
 # BPM 120
 SEQUENCE 0b 0      # Start at beat 0 (0.0s)
-  EFFECT FlashEffect 0b 1b 1     # Flash from beat 0 to beat 1 (0-0.5s)
-  EFFECT HeptagonEffect 4b 8b 0  # Main effect from beat 4 to 8 (2-4s)
+  EFFECT + Flash 0b 1b             # Flash from beat 0 to beat 1 (0-0.5s)
+  EFFECT + Heptagon 4b 8b          # Main effect from beat 4 to 8 (2-4s)
 ```
 
 ---
diff --git a/src/generated/timeline.cc b/src/generated/timeline.cc
index 381c403..0b69b09 100644
--- a/src/generated/timeline.cc
+++ b/src/generated/timeline.cc
@@ -12,35 +12,35 @@ void LoadTimeline(MainSequence& main_seq, WGPUDevice device, WGPUQueue queue, WG
     seq->add_effect(std::make_shared<FlashCubeEffect>(device, queue, format), .2f, 1.500000f, -1);
     seq->add_effect(std::make_shared<FlashEffect>(device, queue, format), 0.0f, 1.f, 0);
     seq->add_effect(std::make_shared<FadeEffect>(device, queue, format), 0.1f, 1.f, 1);
-    seq->add_effect(std::make_shared<SolarizeEffect>(device, queue, format), 0.000000f, 2.000000f, 3);
+    seq->add_effect(std::make_shared<SolarizeEffect>(device, queue, format), 0.000000f, 2.000000f, 2);
     main_seq.add_sequence(seq, 0.000000f, 0);
   }
   {
     auto seq = std::make_shared<Sequence>();
     seq->add_effect(std::make_shared<FlashCubeEffect>(device, queue, format), 0.1f, 3.f, -1);
-    seq->add_effect(std::make_shared<FlashEffect>(device, queue, format), 0.0f, 0.2f, 4);
+    seq->add_effect(std::make_shared<FlashEffect>(device, queue, format), 0.0f, 0.2f, 0);
     main_seq.add_sequence(seq, 2.000000f, 0);
   }
   {
     auto seq = std::make_shared<Sequence>();
     seq->add_effect(std::make_shared<HeptagonEffect>(device, queue, format), 0.0f, .2f, 0);
-    seq->add_effect(std::make_shared<FadeEffect>(device, queue, format), 0.1f, 1.0f, 5);
+    seq->add_effect(std::make_shared<FadeEffect>(device, queue, format), 0.1f, 1.0f, 1);
     main_seq.add_sequence(seq, 3.500000f, 0);
   }
   {
     auto seq = std::make_shared<Sequence>();
     seq->add_effect(std::make_shared<ThemeModulationEffect>(device, queue, format), 0.000000f, 2.000000f, 0);
     seq->add_effect(std::make_shared<HeptagonEffect>(device, queue, format), 0.000000f, 8.000000f, 1);
-    seq->add_effect(std::make_shared<ChromaAberrationEffect>(device, queue, format), 0.000000f, 8.000000f, 3);
-    seq->add_effect(std::make_shared<GaussianBlurEffect>(device, queue, format), 0.000000f, 4.000000f, 4);
+    seq->add_effect(std::make_shared<ChromaAberrationEffect>(device, queue, format), 0.000000f, 8.000000f, 2);
+    seq->add_effect(std::make_shared<GaussianBlurEffect>(device, queue, format), 0.000000f, 4.000000f, 3);
     main_seq.add_sequence(seq, 16.000000f, 0);
   }
   {
     auto seq = std::make_shared<Sequence>();
     seq->add_effect(std::make_shared<ThemeModulationEffect>(device, queue, format), 0.000000f, 2.000000f, 0);
     seq->add_effect(std::make_shared<HeptagonEffect>(device, queue, format), 0.2f, 2.0f, 1);
-    seq->add_effect(std::make_shared<GaussianBlurEffect>(device, queue, format), 0.000000f, 4.000000f, 3);
-    seq->add_effect(std::make_shared<SolarizeEffect>(device, queue, format), 0.000000f, 1.000000f, 5);
+    seq->add_effect(std::make_shared<GaussianBlurEffect>(device, queue, format), 0.000000f, 4.000000f, 2);
+    seq->add_effect(std::make_shared<SolarizeEffect>(device, queue, format), 0.000000f, 1.000000f, 3);
     main_seq.add_sequence(seq, 24.000000f, 0);
   }
   {
@@ -56,13 +56,13 @@ void LoadTimeline(MainSequence& main_seq, WGPUDevice device, WGPUQueue queue, WG
   {
     auto seq = std::make_shared<Sequence>();
     seq->add_effect(std::make_shared<ThemeModulationEffect>(device, queue, format), 0.000000f, 1.500000f, 0);
-    seq->add_effect(std::make_shared<SolarizeEffect>(device, queue, format), 0.000000f, 1.500000f, 5);
+    seq->add_effect(std::make_shared<SolarizeEffect>(device, queue, format), 0.000000f, 1.500000f, 1);
     main_seq.add_sequence(seq, 31.000000f, 0);
   }
   {
     auto seq = std::make_shared<Sequence>();
-    seq->add_effect(std::make_shared<ParticlesEffect>(device, queue, format), 0.000000f, 2.000000f, 1);
-    seq->add_effect(std::make_shared<GaussianBlurEffect>(device, queue, format), 0.000000f, 4.000000f, 1);
+    seq->add_effect(std::make_shared<ParticlesEffect>(device, queue, format), 0.000000f, 2.000000f, 0);
+    seq->add_effect(std::make_shared<GaussianBlurEffect>(device, queue, format), 0.000000f, 4.000000f, 0);
     main_seq.add_sequence(seq, 3.000000f, 1);
   }
   {
@@ -110,9 +110,9 @@ void LoadTimeline(MainSequence& main_seq, WGPUDevice device, WGPUQueue queue, WG
   {
     auto seq = std::make_shared<Sequence>();
     seq->add_effect(std::make_shared<FlashCubeEffect>(device, queue, format), .2f, 1.500000f, -1);
-    seq->add_effect(std::make_shared<GaussianBlurEffect>(device, queue, format), 0.000000f, 4.000000f, 1);
-    seq->add_effect(std::make_shared<FlashEffect>(device, queue, format), 0.0f, 0.2f, 4);
-    seq->add_effect(std::make_shared<FlashEffect>(device, queue, format), 0.500000f, 0.2f, 4);
+    seq->add_effect(std::make_shared<GaussianBlurEffect>(device, queue, format), 0.000000f, 4.000000f, 0);
+    seq->add_effect(std::make_shared<FlashEffect>(device, queue, format), 0.0f, 0.2f, 1);
+    seq->add_effect(std::make_shared<FlashEffect>(device, queue, format), 0.500000f, 0.2f, 1);
     main_seq.add_sequence(seq, 8.000000f, 10);
   }
 }
diff --git a/tools/seq_compiler.cc b/tools/seq_compiler.cc
index 45cfc1a..7ac921f 100644
--- a/tools/seq_compiler.cc
+++ b/tools/seq_compiler.cc
@@ -467,13 +467,52 @@ int main(int argc, char* argv[]) {
                   << ": EFFECT found outside of SEQUENCE\n";
         return 1;
       }
-      std::string class_name, start, end, priority;
-      if (!(ss >> class_name >> start >> end >> priority)) {
+      std::string priority_mod, class_name, start, end;
+      if (!(ss >> priority_mod >> class_name >> start >> end)) {
         std::cerr << "Error line " << line_num
-                  << ": EFFECT requires <Class> <start> <end> <priority>\n";
+                  << ": EFFECT requires <+|=|-> <Class> <start> <end>\n";
         return 1;
       }
 
+      // Validate priority modifier
+      if (priority_mod != "+" && priority_mod != "=" && priority_mod != "-") {
+        std::cerr << "Error line " << line_num
+                  << ": Priority modifier must be '+', '=', or '-', got: " << priority_mod << "\n";
+        return 1;
+      }
+
+      // Calculate priority based on modifier and sequence state
+      static int current_priority = 0;
+      static bool first_in_sequence = true;
+      static const SequenceEntry* last_seq = nullptr;
+
+      // Reset priority tracking for new sequence
+      if (current_seq != last_seq) {
+        current_priority = 0;
+        first_in_sequence = true;
+        last_seq = current_seq;
+      }
+
+      // Handle first effect in sequence
+      if (first_in_sequence) {
+        if (priority_mod == "-") {
+          current_priority = -1;  // Background layer
+        } else {
+          current_priority = 0;   // Default start (+ or =)
+        }
+        first_in_sequence = false;
+      } else {
+        // Update priority based on modifier for subsequent effects
+        if (priority_mod == "+") {
+          current_priority++;
+        } else if (priority_mod == "-") {
+          current_priority--;
+        }
+        // '=' keeps current_priority unchanged
+      }
+
+      std::string priority = std::to_string(current_priority);
+
       // Convert beat notation to time
       std::string start_time = convert_to_time(start, bpm);
       std::string end_time = convert_to_time(end, bpm);