fix(producer): audio drops + blank images on FFmpeg 4.x/CORS-restricted origins (#1140)

* fix(engine): remove amix normalize=0 to fix audio on FFmpeg 4.x/6.x

amix's normalize=0 option is absent from many FFmpeg builds (e.g.
FFmpeg 4.2 on Ubuntu 20.04). When the option is not recognized, FFmpeg
fails the entire filter graph initialization, processCompositionAudio
returns success:false, and the assembled video has no audio stream.

Replace normalize=0 + weights='1...' with the amix default behavior
(normalize=true, divides by track count) and multiply the master output
gain by the track count to restore the original per-track volumes.
The net volume is identical across all FFmpeg versions.

Fixes #1136-adjacent: reported as 'audio doesn't play' in rendered MP4.

* fix(producer): strip img crossorigin + fix audioExtractor normalize=0

Two follow-up fixes:

1. htmlCompiler: strip crossorigin attribute from <img> elements during
   compilation. External images (e.g. S3) with crossorigin='anonymous'
   force CORS-mode requests against the renderer's localhost file server,
   which S3 rejects → images render blank. Matches the existing video
   strip at line 261.

2. audioExtractor: same amix normalize=0 bug as audioMixer.ts. The
   audioExtractor path is used for <video data-has-audio='true'> mixing
   in the CLI's local render pipeline; on FFmpeg 4.x it would also drop
   audio silently. Fix: remove normalize=0, compensate with volume=N.

* test(engine,producer): pin amix normalize contract + img crossorigin strip

- audioMixer.test.ts: assert filter has no normalize=/weights=; add
  3-track test confirming compensatedGain = masterGain × N = 3
- htmlCompiler.test.ts: parallel tests for img and video crossorigin
  strip (covers both elements, not just video)