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, 13 insertions, 13 deletions

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);
   }
 }