Decode tfrag texture bindings, vertex colours and light records

Adds level_core.rs (the uncompressed core index header and texture table)
and extends tfrag.rs with the data a draw call needs beyond geometry.

An ad-gif is five A+D quadwords holding packed fields rather than GS
registers; the level load rewrites them in place before drawing. On disc
TEX0_1.data_lo indexes the level's texture table, TEX1_1.data_lo is the
LOD bias K and data_hi the minification filter, and CLAMP_1 holds both
wrap modes.

Three things are easy to get wrong here:

* K is a signed 12-bit value in 1/16 units, sign-extended by shifting the
  field up 4 and back down. Retail values are -137..-89 mip-scaled;
  reading it unsigned gives 1911 instead of -137.
* TEX1_1.data_hi is MMIN, and 4 decodes as LinearMipmapNearest, not
  LinearMipmapLinear as the raw bit layout suggests.
* RGBA and the light/normal records are not in the VIF lists at all. They
  sit at fixed offsets in the patch data block: rgba_size*4 colours at
  rgba_ofs, vert_count light records at light_ofs+0x10. The tfrag origin
  lives there too -- if no STROW row supplied one, the quadword at
  light_ofs is it.

The texture table checks out on its own terms: 93 entries at index offset
0x2900 on level 0, and 0x2900 + 93*16 lands exactly on the next range.

Verification: an ad-gif index outside the level's table would mean the
geometry and the textures were misread independently, so every one is
range-checked against real entries. All 27 levels: 47,950 ad-gifs over
18,102 patches, all in range, referencing 1429 of 1595 level texture
entries (90%). Each patch's light array is exactly vert_count records and
its RGBA array exactly rgba_size*4.

Still open: LOD selection and morphing, and the texel data itself.

crates/gc-formats/src/lib.rs

@@ -1,6 +1,7 @@
 pub mod buf;
 pub mod global_wad;
 pub mod iso9660;
+pub mod level_core;
 pub mod level_data;
 pub mod level_wad;
 pub mod tfrag;