fix: remove double vsync — page flip already waits for vblank
DRM page flip blocks until vblank (~16ms), then frame_sync_60fps
added ANOTHER 16ms nanosleep on top. Result: 35ms/frame (28fps)
instead of 16ms (60fps). Skip frame_sync when using DRM page flip.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>