Summary

• When combining chunks in distributed rendering (Lambda), the mux step did not specify an explicit framerate (-r), causing ffmpeg to infer a slightly imprecise value from the concatenated stream (e.g. 95940000/3197999 instead of 30/1)
• Added -r <fps> to the mux command in mux-video-and-audio.ts to force the exact output framerate in the container metadata
• No re-encoding occurs — with -c:v copy, the -r flag only sets the mp4 container header timing values

Test plan

• Rendered locally — FPS is 30/1
• Rendered on Lambda with distributed rendering — FPS is now 30/1 (was 95940000/3197999 before fix)

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