fix(udf): read the Metadata File Location from the partition map
read_filesystem hardcoded the Metadata File's File Entry at block 0 of the physical partition — 'the metadata file ICB is at physical partition lba 0'. UDF 2.50 2.2.10 records where it actually lives, as a partition-relative Uint32 at offset 40 of the Metadata Partition Map. That field is the only thing on the volume that says where the entry is; block 0 is merely where authoring tools usually put it. On a conformant volume that recorded it elsewhere, block 0 holds something that is not a File Entry, metadata_start falls back to partition_start, the File Set Descriptor read there carries the wrong tag, and the volume is rejected as UdfNotFilesystem. Worse, a volume with a decoy file set at block 0 — as a rewritten or dual-structure volume can have — does not error at all: it mounts a different filesystem and reports success. Verified on HEAD: reverting the lookup reds four tests, e.g. the metadata partition beginning at 2000 where the map records 33754069. The recorded location is trusted only when the map's partition type identifier reads '*UDF Metadata Partition'. A Virtual (2.2.8) or Sparable (2.2.9) map is ALSO ECMA-167 3/10.7.3 Type 2 and records unrelated fields at offset 40, so its bytes must never be read as a location. Deleting that guard reds its own test. Block 0 stays in the candidate chain, so a volume whose map is absent or wrong but whose Metadata File does sit there keeps mounting exactly as before. This is additive, not a behaviour swap. Also 30 tests and ~55 more mutants across read_icb_extents, read_file_limited, read_inline_data, the prefetch stubs, parse_dstring and parse_udf_name. The metadata-partition branch — the branch EVERY real BD-ROM takes — had no test at all; nothing in the crate built a two-partition-map volume. Closes the max_bytes gap flagged earlier: 259 > -> == and > -> < now die on both the declared-size and the inline-ICB paths. Equivalents proven by application, notably two guards that read as protective but are unreachable: pm1_len is a single byte so 440 + pm1_len < 2048 always holds, and ad_offset + l_ad <= 2048 is enforced upstream so off + ad_size never exceeds the block. Bit 0 (Existence) stays unread, deliberately. ECMA-167 4/14.4.4 makes it a display hint, not a statement that the file is absent, and UDF 2.50 2.3.4.2 carries it through as the DOS hidden attribute. For a ripper the consequences are asymmetric: honouring it can silently drop a real .m2ts from the title list, ignoring it costs an extra name in a listing. Known structural limit, not fixed: read_filesystem takes only the FIRST extent of the Metadata File, so a fragmented metadata partition would map every sector past that extent to the wrong place. metadata_start being a single base LBA is what forbids the fix.
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Matthew Jackson