Parse the level container header, naming each level's four extents

The first typed structure read from real data. Every one of the 27 level
containers begins with the same 0x60-byte header, which turns the generic pair
table into named assets: data, sound_bank, gameplay and occlusion, plus three
chunk descriptors.

Two things about this header are worth stating because both are silent failures
rather than errors.

Sizes here are sector counts, not byte counts. The generic G/ pair table on the
same disc uses byte counts, so both readings type-check and the wrong one is off
by a factor of 2048. The tiling check settles it: read as sectors, the four
extents tile every one of the 27 containers exactly; read as bytes they overlap
each other.

Level id is not container order. LEVEL21.WAD reports id 30, so a level was cut
from the disc and its number left a gap. Anything keying off file order -- a
save slot, the planet select screen -- would mislabel that planet. There is also
a second trap in the same area: the ISO filesystem sorts names alphabetically, so
LEVEL10 precedes LEVEL2, and any tool needing order has to sort numerically.

The header's field at 0x04 is zero on all 27 levels. It is not the container
size despite looking like one, so the code reads it as a runtime field and takes
the container length from the ISO entry instead.

Measured across all 27 levels, three invariants now hold in tests: extents tile
in file order; extent coverage plus chunks reaches at least the end of the
container (21 levels land exactly on its length, 6 leave 134 to 689 sectors of
trailing data); and exactly one level's id differs from its container index. The
coverage assertion is >= rather than == on purpose -- 21 levels end one sector
past the file end because the header occupies sector 0, and asserting equality
would truncate a level's tail.

gameplay is left unparsed and that is recorded rather than hidden. It begins
"WAD" plus a per-level byte and does not decode with the WAD LZ77 decoder: all
27 levels produce plausible output for the first packet and then fail on a rule
the format's own encoder never breaks, which means the input is wrong rather
than the decoder. docs/plan/level-assets.md records what is ruled out and what
to try next.

66 unit tests, plus 7 integration tests against the real disc. The unpack hash
table is unchanged, so this adds parsing without altering the pipeline.

crates/gc-formats/src/lib.rs

@@ -1,4 +1,5 @@
 pub mod buf;
 pub mod global_wad;
 pub mod iso9660;
+pub mod level_wad;
 pub mod wad;