[Nawid3333]

Follow-up to the dependency audit: the one piece of other projects' code that no update workflow watched was the save's MP4 writer. MP4Generator.mjs started as a copy of hls.js's mp4-generator. This PR puts it on the normal hls.js update path instead of a separate watcher.

What changes

Compared function by function with hls.js 1.7.4, 15 of its functions were still hls.js's code, unchanged.

• Taken from hls.js itself:
  • The video sample entries (avc1, hvc1) come from hls.js's own MP4, exported through one more name in the hls.js patch's single export line. hls2mp4 writes them and hands them over as the track's codecBuffer, the pass-through a DASH track already uses.
  • The fragment boxes (moof, traf, trun, mfhd, mvex, trex, mdat) were used only by the tests' fMP4 helper; it now uses hls.js's.
  • MP4Generator: 966 → 696 lines. hls.js's fixes to these now arrive with each hls.js update PR.
• Checked against hls.js: mp4WriterUpstream.test.mjs fingerprints each hls.js function FastStream keeps its own version of. That's the containers, sample tables, tkhd, the audio entries and normalizePts, with comments and whitespace ignored. An hls.js update that changes one fails in that update's own PR (twice red → hold + a "Migration" issue), and the message says what to compare.
• Guards:
  • The patch must export MP4.
  • No DASH save module may import hls.js. Importing it in MP4Generator itself would make every DASH save load hls.js's 1.2 MB, so the sample entry is built in hls2mp4, which has hls.js loaded already.

Why not take over all 15 unchanged functions: hls.js's functions call each other as MP4.x. Its minf or moov would write its own empty sample table and fragment header into a plain MP4 file.

Proof that nothing changes

• Byte comparison: every box the writer returned across the 49 save unit tests (hls2mp4, dash2mp4, mp4merger, saveDownloadsAhead: H.264, HEVC, AAC, MP3, AC-3, fMP4) was hashed before and after. All 1,745 boxes FastStream's writer produces are byte-identical, including 54 whole file headers and 99 stsd. All 178 boxes now written by hls.js are there again unchanged.
• Local checks: lint, typecheck, unit (3424) green. Save e2e: save-video, save-fmp4, archive-roundtrip, save-flow, save-transport, save-dialog-regression, all passed.
• Reviews: glm and I designed this together (option B + fingerprints). glm then reviewed the diff and found no regression path.

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