Parse the sound bank container framing

A level's sound extent is two regions: an `SBlk` descriptor table and the
sample data that follows it. Four u32 at the head describe the layout, and
the invariant that establishes it is sblk_offset + sblk_size == daw_offset,
which holds on all 27 levels. That is what pins the field meanings; the
numbers alone are ambiguous about which of them is the offset.

DAW is a format tag inside the SBlk header, not a region signature. It
occurs exactly once in the entire extent, at sblk_offset + 12, naming the
sample format -- the sample data itself starts at daw_offset with no
signature at all, roughly 19 KB away on level 0. Reading it as a region
magic is the obvious reading and it is wrong.

daw_size never reaches the end of the file. The tail is positive on all 27
levels, 232 to 1,848 bytes, so the field describes less than the region
it names; the test asserts that direction rather than inventing a size
that fits.

523,720 SBlk and 37,649,008 DAW bytes across the 27 levels.

The sample format inside DAW is not decoded. It looks like VAG/ADPCM from
the nibble packing, and it has no bearing on drawing anything, so it stays
open rather than getting a half-answer.

crates/gc-formats/src/lib.rs

@@ -5,6 +5,7 @@ pub mod level_core;
 pub mod level_data;
 pub mod level_wad;
 pub mod occlusion;
+pub mod sound_bank;
 pub mod texture;
 pub mod tfrag;
 pub mod tfrag_light;