Eyevinn / mp4ff / 37909981006

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fix(mp4): look up the stsc sample description ID by chunk

GetSampleDescriptionID takes a 1-based chunk number, as its name and doc say, but used it as an index into the per-entry SampleDescriptionID slice. For an stsc with several sample description IDs, it returned the ID of the entry with that number, and 0 for chunk numbers beyond the number of entries. It now finds the entry that the chunk belongs to, as GetChunk does: of entries sharing a firstChunk the last one applies, and the last entry covers all later chunks.

Info, which lists the entries, and the defragmenter, which walks them, read the ID per entry through the new unexported entrySampleDescriptionID.

When cropping an stsc with several IDs, mp4ff-crop truncated Entries twice where it meant to truncate SampleDescriptionID. The IDs of the dropped entries stayed, so an entry added for a partly kept chunk was encoded with the ID of the first dropped entry. When the crop ended in the last entry, the getter returned 0, AddEntry stored no ID, and encoding panicked with index out of range.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

13 of 13 new or added lines in 3 files covered. (100.0%)

23854 of 28823 relevant lines covered (82.76%)

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