Implement non-destructive box editing and enhance decoder documentation#7
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Supersedes the PR #5 splice-based prototype with an extent-tree design whose two correctness gaps (stale ancestor sizes, heuristic chunk-offset fixup) are impossible by construction. Design: - edit/tree.rs: the file parses into a tree where unmodified boxes are byte-range references into the source, not copies. Container prefix bytes (stsd entry_count, sample-entry fixed headers), padding, and original header forms (compact/largesize/to-EOF) are preserved. Serialization streams extents in chunks (mdat never held in memory) and recomputes box sizes bottom-up, so every ancestor of an edited box is correct by construction. Invariant: serializing an unedited tree reproduces the source byte for byte. - edit/fixup.rs: layout produces an exact old->new extent map; each stco/co64 entry is remapped through the extent containing it. No heuristics: unmoved data keeps its offsets, moved data shifts by exactly its extent's delta. Entries pointing at removed data are left unchanged and reported. - edit/fields.rs: Set patches named fields (mvhd/tkhd/mdhd timestamps, timescale, duration, rate, volume, track_id, layer, width/height, language) in place with version-aware offsets; matrices, reserved regions, and unknown bytes are untouched — no re-encoding. - edit/mod.rs + tags.rs: Editor with Remove/RemoveAll/Insert/Replace/ Set/SetTag commands; slash paths with [n] indexing and © fourccs; SetTag creates the moov/udta/meta/ilst chain (ffmpeg-compatible hdlr) when missing. Fragmented and HEIF files (moof/sidx/iloc) are refused with a clear error since their internal offsets are not yet covered by fixup; no-op pass-through still works. Packaging: "edit" cargo feature (on by default; disable for a parse-only build), mp4edit binary (--remove/--remove-all/--insert/ --replace/--set/--tag), and examples/edit.rs. Tests (tests/edit_roundtrip.rs): byte-identical zero-edit round-trips on synthetic and real files; removal before mdat shifts stco entries by exactly the removed size while removal after mdat leaves them untouched; samples still resolve to identical media bytes after every edit; ancestor sizes re-tile the file with no gaps; Set changes only the addressed bytes; tag create/replace; fragmented-file guard. Verified on a real file with ffprobe/ffmpeg: tags readable, all 2336 chunk offsets adjusted, zero decode errors.
The extension point already existed (get_boxes_with_registry + Registry::with_decoder) but was invisible enough that PR #3 was opened to add what was already there. No new functionality: - add a worked doc example on get_boxes_with_registry showing the intended pattern: extend the default registry with a custom decoder (default_registry().with_decoder(...)) rather than replace it - re-export default_registry at the crate root - add an integration test proving a custom decoder fires through get_boxes_with_registry alongside the built-in decoders Supersedes PR #3.
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