fix(build): Resolve clean build failure from generated timeline header

When starting from a clean tree (where `src/generated/` does not exist), the build would fail with a "file not found" error for `generated/timeline.h`. This was due to an incorrect dependency graph where `gpu.cc` was compiled before its required header was generated.

This commit fixes the issue by making the dependency explicit:

- Modified `tools/seq_compiler.py` to explicitly generate `timeline.h` alongside `timeline.cc`.
- Updated `cmake/DemoCodegen.cmake` to declare both files as `OUTPUT`s of the timeline compilation step.
- Added a direct dependency from the `gpu` library target to the `generate_timeline` custom target in `cmake/DemoLibraries.cmake`.
- Refactored the generated file paths in `DemoCodegen.cmake` into a single `GENERATED_CODE` variable for improved clarity and future-proofing.

3 files changed, 29 insertions, 7 deletions

diff --git a/cmake/DemoCodegen.cmake b/cmake/DemoCodegen.cmake
index 1e0badf..aa20a92 100644
--- a/cmake/DemoCodegen.cmake
+++ b/cmake/DemoCodegen.cmake
@@ -122,15 +122,17 @@ endfunction()
 # Timeline compilation
 set(DEMO_SEQ_PATH ${WORKSPACE_TIMELINE})
 set(GENERATED_TIMELINE_CC ${CMAKE_CURRENT_SOURCE_DIR}/src/generated/timeline.cc)
+set(GENERATED_TIMELINE_H ${CMAKE_CURRENT_SOURCE_DIR}/src/generated/timeline.h)
+set(GENERATED_CODE ${GENERATED_TIMELINE_CC} ${GENERATED_TIMELINE_H})
 add_custom_command(
-    OUTPUT ${GENERATED_TIMELINE_CC}
+    OUTPUT ${GENERATED_CODE}
     COMMAND ${CMAKE_COMMAND} -E make_directory ${CMAKE_CURRENT_SOURCE_DIR}/src/generated
     COMMAND ${SEQ_COMPILER_CMD} ${DEMO_SEQ_PATH} --output ${GENERATED_TIMELINE_CC}
     DEPENDS ${SEQ_COMPILER_DEPENDS} ${DEMO_SEQ_PATH}
             src/gpu/demo_effects.h
     COMMENT "Compiling demo sequence from workspace ${DEMO_WORKSPACE}..."
 )
-add_custom_target(generate_timeline ALL DEPENDS ${GENERATED_TIMELINE_CC})
+add_custom_target(generate_timeline ALL DEPENDS ${GENERATED_CODE})
 
 # Music compilation
 set(TRACKER_MUSIC_PATH ${WORKSPACE_MUSIC})
@@ -255,5 +257,5 @@ pack_test_assets(test_assets ${CMAKE_CURRENT_SOURCE_DIR}/workspaces/test/assets.
 # Mark generated files so CMake always checks if they need rebuilding
 set_source_files_properties(${GEN_DEMO_H} ${GEN_DEMO_CC} PROPERTIES GENERATED TRUE)
 set_source_files_properties(${GEN_TEST_H} ${GEN_TEST_CC} PROPERTIES GENERATED TRUE)
-set_source_files_properties(${GENERATED_TIMELINE_CC} PROPERTIES GENERATED TRUE)
+set_source_files_properties(${GENERATED_CODE} PROPERTIES GENERATED TRUE)
 set_source_files_properties(${GENERATED_MUSIC_DATA_CC} PROPERTIES GENERATED TRUE)
diff --git a/cmake/DemoLibraries.cmake b/cmake/DemoLibraries.cmake
index f1891fb..39cd3f4 100644
--- a/cmake/DemoLibraries.cmake
+++ b/cmake/DemoLibraries.cmake
@@ -21,9 +21,9 @@ target_include_directories(3d PUBLIC ${CORE_INCLUDES})
 add_dependencies(3d generate_demo_assets)
 
 # GPU effects library
-add_library(gpu STATIC ${GPU_SOURCES})
+add_library(gpu STATIC ${GPU_SOURCES} ${GENERATED_TIMELINE_CC})
 target_include_directories(gpu PUBLIC ${CORE_INCLUDES})
-add_dependencies(gpu generate_demo_assets)
+add_dependencies(gpu generate_demo_assets generate_timeline)
 
 # Note: Static libraries do not strictly need to link dependencies,
 # but if they did, PRIVATE would propagate to the executable.
diff --git a/tools/seq_compiler.py b/tools/seq_compiler.py
index dec3ab1..18d5a1f 100755
--- a/tools/seq_compiler.py
+++ b/tools/seq_compiler.py
@@ -710,11 +710,31 @@ void RenderTimeline(WGPUSurface surface, float time, int width, int height,
 }
 '''
 
-    # Write output
+    # Write C++ output
     with open(args.output, 'w') as f:
         f.write(all_cpp)
 
-    print(f"Generated {len(sequences)} sequence(s) -> {args.output}")
+    # Write C++ header
+    header_path = os.path.splitext(args.output)[0] + ".h"
+    header_content = f'''// Generated by seq_compiler.py
+// DO NOT EDIT
+#pragma once
+
+#include "gpu/gpu.h"
+#include "gpu/sequence.h"
+
+void InitializeSequences(const GpuContext& ctx, int width, int height);
+Sequence* GetActiveSequence(float time);
+void RenderTimeline(WGPUCommandEncoder encoder, float time, int width, int height,
+                      float beat_time, float audio_intensity);
+void RenderTimeline(WGPUSurface surface, float time, int width, int height,
+                      float beat_time, float audio_intensity);
+float GetDemoDuration();
+'''
+    with open(header_path, 'w') as f:
+        f.write(header_content)
+
+    print(f"Generated {len(sequences)} sequence(s) -> {args.output} and {header_path}")
 
 if __name__ == '__main__':
     main()