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.

1 files changed, 56 insertions, 54 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