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feat(mp4): generate ftyp and styp brands from the content

InitSegment.GenerateFtyp computes the lowest brand of the ISO/IEC 14496-12:2026 Annex E chain that supports the boxes of the init segment and the declared contents of its media segments (FragmentFeatures, defaulting to what mp4ff's fragment writer emits). The ftyp must be written before the segments exist, which is why their contents are declared. It adds av01 and iamf, which their codec specifications require, and dby1 for Dolby Vision. With CMAF requested, cmfc or cmf2 becomes the major brand with minor version 0, and a CMAF header with more than one track, box versions beyond iso9 (23000-19 7.3.1), or fragments without tfdt or default-base-is-moof is rejected. Otherwise the major brand is mp42, since Annex E brands should not be the major brand (E.1). The major brand is always repeated (5.2).

MediaSegment.GenerateStyp adds msdh when the segment has the DASH Simple format (23009-1 6.3.5.3), msix for the Indexed format (6.3.5.4), cmfs, cmff and cmfl plus cmfr at a sync sample for CMAF segments, and lmsg for the last segment. SetFtyp replaces the ftyp of an init segment.

CreateFtyp and CreateStyp repeat the major brand, drop dash, which declares a self-initializing indexed segment in DASH and has no meaning in a live init or in styp, and CreateStyp adds cmff and cmfl, which a CMAF segment start carries (23000-19 7.3.2.4 NOTE 2). The muxed mode of the segmenter example now uses the generator, since a multi-track init is not a CMAF header.

Refs #540

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017q3eYmMQXKrP3iY8UCS48r

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