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, 36 insertions, 17 deletions

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)
 ```
 
 ---