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>

3 files changed, 18 insertions, 35 deletions

diff --git a/cmake/DemoExecutables.cmake b/cmake/DemoExecutables.cmake
index e3337d7..5131936 100644
--- a/cmake/DemoExecutables.cmake
+++ b/cmake/DemoExecutables.cmake
@@ -47,15 +47,16 @@ endif()
 # test_demo - Audio/Visual Sync Tool
 # =============================================================================
 
-# Timeline generation
+# Timeline generation (v2)
 set(TEST_DEMO_SEQ_PATH ${CMAKE_CURRENT_SOURCE_DIR}/tools/test_demo.seq)
-set(GENERATED_TEST_DEMO_TIMELINE_CC ${CMAKE_CURRENT_SOURCE_DIR}/src/generated/test_demo_timeline.cc)
+set(GENERATED_TEST_DEMO_TIMELINE_CC ${CMAKE_CURRENT_SOURCE_DIR}/src/generated/test_timeline.cc)
 add_custom_command(
     OUTPUT ${GENERATED_TEST_DEMO_TIMELINE_CC}
     COMMAND ${CMAKE_COMMAND} -E make_directory ${CMAKE_CURRENT_SOURCE_DIR}/src/generated
-    COMMAND ${SEQ_COMPILER_CMD} ${TEST_DEMO_SEQ_PATH} ${GENERATED_TEST_DEMO_TIMELINE_CC}
-    DEPENDS ${SEQ_COMPILER_DEPENDS} ${TEST_DEMO_SEQ_PATH} src/gpu/demo_effects.h
-    COMMENT "Compiling test_demo sequence..."
+    COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/tools/seq_compiler_v2.py
+            ${TEST_DEMO_SEQ_PATH} --output ${GENERATED_TEST_DEMO_TIMELINE_CC}
+    DEPENDS ${TEST_DEMO_SEQ_PATH}
+    COMMENT "Compiling test_demo sequence (v2)..."
 )
 add_custom_target(generate_test_demo_timeline ALL DEPENDS ${GENERATED_TEST_DEMO_TIMELINE_CC})
 
@@ -74,6 +75,7 @@ add_custom_target(generate_test_demo_music ALL DEPENDS ${GENERATED_TEST_DEMO_MUS
 # Mark test_demo generated files as GENERATED
 set_source_files_properties(${GENERATED_TEST_DEMO_TIMELINE_CC} PROPERTIES GENERATED TRUE)
 set_source_files_properties(${GENERATED_TEST_DEMO_MUSIC_CC} PROPERTIES GENERATED TRUE)
+set_source_files_properties(${CMAKE_CURRENT_SOURCE_DIR}/src/generated/test_timeline_v2.h PROPERTIES GENERATED TRUE)
 
 # Build executable (uses main demo assets)
 if(NOT DEMO_STRIP_EXTERNAL_LIBS)
diff --git a/cmake/DemoSourceLists.cmake b/cmake/DemoSourceLists.cmake
index 6b56206..f142ce1 100644
--- a/cmake/DemoSourceLists.cmake
+++ b/cmake/DemoSourceLists.cmake
@@ -28,7 +28,6 @@ set(UTIL_SOURCES src/util/asset_manager.cc src/util/file_watcher.cc)
 
 # Common effect sources (shared between headless and normal modes)
 set(COMMON_GPU_EFFECTS
-    src/gpu/effect.cc
     src/gpu/sequence_v2.cc
     src/gpu/effect_v2.cc
     src/effects/passthrough_effect_v2.cc
@@ -38,31 +37,14 @@ set(COMMON_GPU_EFFECTS
     src/effects/particles_effect_v2.cc
     src/effects/rotating_cube_effect_v2.cc
     src/effects/hybrid3_d_effect_v2.cc
-    src/effects/heptagon_effect.cc
-    src/effects/particles_effect.cc
-    src/effects/passthrough_effect.cc
-    src/effects/moving_ellipse_effect.cc
-    src/effects/particle_spray_effect.cc
-    src/effects/gaussian_blur_effect.cc
-    src/effects/solarize_effect.cc
-    src/effects/scene1_effect.cc
-    src/effects/chroma_aberration_effect.cc
-    src/effects/vignette_effect.cc
-    cnn_v1/src/cnn_v1_effect.cc
-    cnn_v2/src/cnn_v2_effect.cc
+    # TODO: Port CNN effects to v2 (complex v1 dependencies)
+    # cnn_v1/src/cnn_v1_effect.cc
+    # cnn_v2/src/cnn_v2_effect.cc
     src/gpu/post_process_helper.cc
     src/gpu/shaders.cc
-    src/effects/hybrid_3d_effect.cc
-    src/effects/flash_cube_effect.cc
-    src/effects/theme_modulation_effect.cc
-    src/effects/fade_effect.cc
-    src/effects/flash_effect.cc
     src/gpu/shader_composer.cc
-    src/effects/circle_mask_effect.cc
-    src/effects/rotating_cube_effect.cc
     src/gpu/texture_manager.cc
     src/gpu/texture_readback.cc
-    src/effects/sdf_test_effect.cc
 )
 
 # GPU sources (conditional: HEADLESS / STRIP_EXTERNAL / NORMAL)
diff --git a/cmake/Validation.cmake b/cmake/Validation.cmake
index 328357e..ee150f4 100644
--- a/cmake/Validation.cmake
+++ b/cmake/Validation.cmake
@@ -9,19 +9,18 @@ add_custom_target(validate_uniforms_script ALL DEPENDS ${CMAKE_CURRENT_SOURCE_DI
 # Find all WGSL files recursively in src/gpu
 file(GLOB WGSL_FILES RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_CURRENT_SOURCE_DIR}/src/gpu/*.wgsl ${CMAKE_CURRENT_SOURCE_DIR}/src/gpu/**/*.wgsl)
 
-# List of C++ files containing uniform struct definitions and shader code
+# List of C++ files containing uniform struct definitions and shader code (v2)
 # Add more C++ files here if new effects with structs are added.
 set(VALIDATION_CPP_FILES
-    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/heptagon_effect.cc
     ${CMAKE_CURRENT_SOURCE_DIR}/src/gpu/post_process_helper.h
-    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/fade_effect.cc
-    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/theme_modulation_effect.cc
-    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/chroma_aberration_effect.cc
-    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/vignette_effect.cc
-    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/gaussian_blur_effect.cc
-    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/distort_effect.cc
     ${CMAKE_CURRENT_SOURCE_DIR}/src/gpu/demo_effects.h
-    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/circle_mask_effect.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/heptagon_effect_v2.cc
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/gaussian_blur_effect_v2.cc
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/passthrough_effect_v2.cc
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/placeholder_effect_v2.cc
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/particles_effect_v2.cc
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/rotating_cube_effect_v2.cc
+    ${CMAKE_CURRENT_SOURCE_DIR}/src/effects/hybrid3_d_effect_v2.cc
 )
 
 # Add custom command to run the validator