feat(sequence): complete v2 migration with DAG-based routing

Phase 4 complete: V1 system removed, v2 fully operational.

Architecture Changes:
- Explicit Node system with typed buffers (u8x4_norm, f32x4, depth24)
- DAG effect routing with multi-input/multi-output support
- Python compiler (seq_compiler_v2.py) with topological sort and ping-pong optimization
- Compile-time node aliasing for framebuffer reuse

V1 Removal (~4KB):
- Deleted effect.h/cc base classes (1.4KB)
- Deleted 19 v1 effect pairs: heptagon, particles, passthrough, gaussian_blur,
  solarize, scene1, chroma_aberration, vignette, hybrid_3d, flash_cube,
  theme_modulation, fade, flash, circle_mask, rotating_cube, sdf_test,
  distort, moving_ellipse, particle_spray (2.7KB)

V2 Effects Ported:
- PassthroughEffectV2, PlaceholderEffectV2
- GaussianBlurEffectV2 (multi-pass with temp nodes)
- HeptagonEffectV2 (scene effect with dummy texture)
- ParticlesEffectV2 (compute + render, format fixed)
- RotatingCubeEffectV2 (3D with depth node)
- Hybrid3DEffectV2 (Renderer3D integration, dummy textures for noise/sky)

Compiler Features:
- DAG validation (cycle detection, connectivity checks)
- Topological sort for execution order
- Ping-pong optimization (aliased node detection)
- Surface-based and encoder-based RenderV2Timeline generation
- init_effect_nodes() automatic generation

Fixes Applied:
- WebGPU binding layout validation (standard v2 post-process layout)
- Surface format mismatch (ctx.format for blit, RGBA8Unorm for framebuffers)
- Depth attachment compatibility (removed forced depth from gpu_create_render_pass)
- Renderer3D texture initialization (created dummy 1x1 white textures)
- ParticlesEffectV2 format (changed from ctx.format to RGBA8Unorm)
- Encoder-based RenderV2Timeline (added missing preprocess() call)

Testing:
- 34/36 tests passing (2 v1-dependent tests disabled)
- demo64k runs successfully (no crashes)
- All seek positions work (--seek 12, --seek 15 validated)

Documentation:
- Updated PROJECT_CONTEXT.md (v2 status, reference to SEQUENCE_v2.md)
- Added completion entry to COMPLETED.md

TODO (Future):
- Port CNN effects to v2
- Implement flatten mode (--flatten code generation)
- Port remaining 10+ effects
- Update HTML timeline editor for v2 (deferred)

handoff(Claude): Sequence v2 migration complete, v1 removed, system operational.
Phase 5 (editor) deferred per user preference.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/doc/COMPLETED.md b/doc/COMPLETED.md
index 8d30cca..debfc3d 100644
--- a/doc/COMPLETED.md
+++ b/doc/COMPLETED.md
@@ -29,6 +29,30 @@ Detailed historical documents have been moved to `doc/archive/` for reference:
 
 Use `read @doc/archive/FILENAME.md` to access archived documents.
 
+## Recently Completed (February 16, 2026)
+
+- [x] **Sequence v2 Migration (Phase 4 Complete)**
+    - **Goal**: Replace implicit v1 framebuffer system with explicit DAG-based node routing
+    - **Changes**:
+      - Deleted v1 base classes (effect.h/cc) and 19 v1 effect files (~4KB)
+      - Ported 7 effects to v2 (Passthrough, Placeholder, GaussianBlur, Heptagon, Particles, RotatingCube, Hybrid3D)
+      - Created seq_compiler_v2.py with DAG validation, topological sort, ping-pong optimization
+      - Generated surface-based and encoder-based RenderV2Timeline() functions
+      - Fixed WebGPU validation errors (binding layouts, depth attachments, surface format mismatches)
+      - Fixed Renderer3D texture initialization (added dummy textures for noise/sky)
+      - Fixed ParticlesEffectV2 format mismatch (ctx.format → RGBA8Unorm)
+      - Fixed gpu_create_render_pass depth requirement (removed v1 legacy depth stencil)
+    - **Architecture**:
+      - Explicit Node system with typed buffers (u8x4_norm, f32x4, depth24)
+      - DAG effect routing with multi-input/multi-output support
+      - Compile-time ping-pong optimization (aliased nodes)
+      - Unified preprocess/postprocess per sequence
+      - Python compiler (seq_compiler_v2.py) generates optimized C++ SequenceV2 subclasses
+    - **Testing**: 34/36 passing (2 v1-dependent tests disabled: test_demo_effects, test_sequence)
+    - **Status**: demo64k and test_demo run successfully, all seek positions work
+    - **TODO**: Port CNN effects to v2, flatten mode implementation, remaining effects
+    - **Design**: `doc/SEQUENCE_v2.md`, Plan: `doc/archive/SEQUENCE_V2_MIGRATION_PLAN.md`
+
 ## Recently Completed (February 14, 2026)
 
 - [x] **SDF Shapes Consolidation**