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.

1 files changed, 22 insertions, 2 deletions

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()