Decode level textures: 8-bit indices through a CSM1 palette

Adds texture.rs and makes the pipeline write gs_ram.bin, which it had
been skipping. Nothing could be decoded before this: the CLUTs live
there.

A level texture is not pixels, it is palette indices -- width*height
bytes at core_data[textures_base + data_offset], with colour coming from
a 256-entry CLUT at gs_ram[palette * 0x100]. gs_ram is a byte range in
the level's data extent header at offset 0x10, 518,656 bytes on level 0.

Two PS2 conventions have to be honoured and neither is a design choice:
within each 32-entry CLUT group the two middle 8-entry blocks are
swapped (bits 3 and 4 of the index are exchanged, an involution), and
alpha runs 0..=0x80 rather than 0..=255 with 0x80 fully opaque.

The mip chain splits across two buffers: levels 0 and 1 are in the
texture block with level 1 immediately after level 0, and levels 2 and 3
are resident in gs_ram at mipmap*0x100 and pad*0x100. Tfrag textures are
square so every level halves evenly.

Verification is two-layered, because every step here is a table lookup
and a wrong CLUT remap still produces a well-formed image of the wrong
thing. All 1,595 tfrag textures across all 27 levels decode; 96.8% are
fully opaque and 96.3% use more than eight colours. On top of that,
`gc-extract dump-texture` writes any texture or mip level as a PPM.
Level 0 texture 0 comes back as a clean bevelled metal panel with rivets
and specular highlights, and texture 74 as a vent grille whose punched
windows are fully transparent. That tool is worth keeping: it is the
cheapest way to tell a decode that is merely well-formed from one that
is right.

The hash table is regenerated for the 27 new gs_ram.bin files, now 5,186
files and 5,685,541,143 bytes.

docs/roadmap.md

@@ -14,7 +14,7 @@ early, while they are still cheap to retire.
 - [x] Repository hygiene in place (pre-commit guard, `.gitignore`, licence
       record)
 - [x] **Phase 1 complete.** The disc pipeline runs end to end and is verified
-      against a committed hash table: 5,135 files, 4,500,106,960 bytes, every
+      against a committed hash table: 5,186 files, 5,685,541,143 bytes, every
       one of the 92 asset containers parsing.
 - [x] **Phase 2 in progress.** Level containers are parsed through their typed
       header, and the `data` extent is parsed as a container of byte ranges. The
@@ -134,6 +134,13 @@ Phase 1 hands over the extents; this phase works out what each one contains.
       in. Per-vertex RGBA and light/normal records recovered from their fixed
       offsets. All 27 levels: 47,950 ad-gifs, every index in range, 90% of the
       level texture tables referenced.
+- [x] **Level textures decode.** 8-bit indexed texels plus a 256-entry CSM1
+      CLUT from `gs_ram`, honouring the PS2 CLUT block swap and the 0x80 alpha
+      scale. The pipeline now writes `gs_ram.bin`, which it had been skipping.
+      All 1,595 tfrag textures on all 27 levels decode; 96.8% fully opaque and
+      96.3% with real detail. Verified visually via the new
+      `gc-extract dump-texture`, because a wrong lookup still yields a
+      well-formed image of the wrong thing.
 - [ ] tfrag LOD selection: the switch and morph rules in `TfragProc`.
 - [ ] `gameplay` — begins `WAD` + a per-level byte, and does *not* decode with
       the WAD LZ77 decoder. All 27 levels fail identically, so the input is