Eyevinn / mp4ff / 36385869677

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fix(mp4): pick the media header of an empty track from its handler type

CreateEmptyTrak chose the media header from the mediaType string, and CreateHdlr chose the handler from the same string, but the two lists of aliases differed. As a result:

- "subt" and "clcp" gave a subt handler with nmhd, although ISO/IEC 14496-12:2026 Section 12.6.2 and CMAF 23000-19 Section 7.5.8 require sthd for subtitle tracks. livesim2 creates its stpp tracks with "subt".
- "stpp" gave sthd but the handler type 'stpp', which is not a handler type.
- "subtitles" panicked, since CreateHdlr rejected it.
- "soun" gave smhd but left the tkhd volume at 0.

CreateHdlr now accepts "subtitles" and "stpp" for the subt handler, and CreateEmptyTrak derives both the media header and the audio volume from the handler type that CreateHdlr returns.

Refs #540

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

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