Parse MKV lacing, route by real TrackNumber, honour NAL length size and edit lists
Four conformance defects in the read paths, two of them silent corruption. **Lacing was ignored entirely.** RFC 9559 §10.2 defines Xiph, EBML and fixed-size lacing, where one Block carries several frames; the reader took the Block payload verbatim, so a laced Block became a single "frame" consisting of a lacing header followed by concatenated frames — garbage to the codec parser, no error. Audio tracks from other muxers commonly use lacing, so an ordinary foreign MKV was silently mangled. All three modes are now parsed: Xiph 255-run sizes including the trailing-zero rule for exact multiples of 255, EBML unsigned first size plus SIGNED VINT deltas with the 2^((7*n)-1)-1 bias of §10.3.3, and fixed-size even division, with the last frame's size deduced from the remainder. Laced timestamps follow §10.3.5: the first frame takes the Block timestamp and the rest are spaced by the track's DefaultDuration, else BlockDuration/count, else shared with a warn. Parsing was chosen over refusing because refusal would leave freemkv unable to remux common foreign audio at all, and each mode is about fifteen lines. A malformed lacing header now raises a NEW code, E_MKV_LACING_INVALID = 9052, deliberately NOT MkvInvalid — because is_skippable_title_stub classifies MkvInvalid as a skippable nav stub, so reusing it would have recreated the exact conflation that is still open as a separate finding. A test asserts the new code is not skippable. **TrackNumber was assumed to be 1..N in TrackEntry order.** RFC 9559 §5.1.4.1.1 only requires it to be non-zero and unique, so sparse or unordered numbers are legal. Block routing and codec_private both computed track + 1. A real TrackNumber map is now built, recorded only for TrackEntries that yield a stream so dropped track types no longer shift the mapping. Verified red here independently, and the failure mode is worse than mis-routing: with track + 1 restored, a buttons track's payload was attributed to the AUDIO stream — wrong payload into the wrong codec parser. **The NAL length prefix was hardcoded to 4 bytes.** lengthSizeMinusOne lives in avcC byte 4 and hvcC byte 21 (ISO/IEC 14496-15 §5.3.3.1.2, §8.3.3.1.2) and was never read, so a source declaring 1- or 2-byte prefixes had its raw prefixed bytes emitted verbatim with no start codes. All four conversion sites now derive the width from the track's own configuration record. **Edit lists were ignored.** No edts/elst was parsed, so the presentation timeline an edit list defines (ISO/IEC 14496-12 §8.6.5/§8.6.6) was dropped — which is how encoder delay is normally expressed. Leading empty edits and the first media edit's media_time are now applied to both dts and pts, with the movie vs media timescale distinction respected. A list needing more than a constant shift applies the leading edit and warns rather than presenting the result as faithful. 17 tests. I reproduced the lacing mutant independently: returning the body whole kills five of them, including the exact-payload and malformed-header cases. Still open and deliberately untouched: the MkvInvalid / is_skippable_title_stub conflation across ~20 reader raise sites. It is a cross-cutting error.rs change and E_MKV_LACING_INVALID is the template for it.
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Matthew Jackson