[Btoala]

Documented edit_decisions.audio.music.asset_id configurations could render without background music because asset resolution, staging, and the composition prop shapes did not connect. This change resolves the track against the manifest and sends a playable source and translated fade settings to the existing CinematicRenderer and Explainer compositions.

Changes

• Support canonical audio.music, legacy top-level music, and existing direct source formats through the Remotion adapter.
• Reject unknown asset IDs, missing local files, and invalid volume/timing values before launching the render CLI.
• Preserve explicit mute, composition-specific defaults, narration/SFX payloads, captions, and video data.
• Fix zero-second fades in both compositions and bump the tool version to invalidate cached silent output.
• Add 80 passing offline regression cases, six opt-in full-render cases, and configuration documentation.

Adapted from upstream OpenMontageInc#938 (issue OpenMontageInc#937). The upstream FFmpeg music-to-audio_path change is intentionally excluded: that path replaces existing dialogue/SFX and bypasses music volume/fades. FFmpeg, HyperFrames, atelier routing, and existing complete-mix behavior are unchanged.

Independently verified

• Exact upstream patch and its five reported test files: 57 passed.
• Adapted local regression selection: 219 passed, 6 skipped (80 new passing cases plus 139 existing related cases; overlaps the upstream run).
• Six focused regression cases fail against the original base as expected.
• Two four-second FFmpeg diagnostics made from the staged music source and actual JSX envelope calculations passed decoded-audio checks: expected gain, audible fades, and retained synthetic narration/SFX tones.
• Existing audio mux/mixing, caption props, staging cleanup, video transitions, profile/palette, and cache regressions passed.
• make lint, changed Python compilation, Node helper syntax, and patch whitespace/application checks passed.

Required before merge

A full Remotion video render is not verified. The first local browser test aborted because Chromium could not create a required local socket: socket() failed: Operation not permitted (1) (SIGABRT). The remaining browser tests were not run; the six opt-in cases are skipped in the normal suite. The FFmpeg diagnostics are intermediate-contract evidence, not end-to-end Remotion renders.

On a host with existing dependencies and working Chromium, run:

OPENMONTAGE_TEST_REMOTION=1 REMOTION_TEST_BROWSER=/path/to/chromium \
  python -m pytest -q tests/tools/test_video_compose_music_render.py

Keep this PR in draft until those tests pass. No generation APIs, publishing, paid credits, or dependency installations were used for the local verification.