docs(re): CONFIRM T8aD colour interpretation (linear UNORM, ARGB order)

Resolved the "colours look odd" channel-order/sRGB question for k_8_8_8_8
(T8aD 2D UI textures) with no emulator run:

- sRGB: Canary's Vulkan host-format table maps plain k_8_8_8_8 to
  R8G8B8A8_UNORM and contains no R8G8B8A8_SRGB anywhere — gamma is a
  separate explicit EDRAM path. So these are raw linear bytes; apply no
  sRGB/gamma decode.
- Channel order: measured 789 real disc textures — byte0 is the opaque
  alpha (==0xFF-dominant) in 97% — so on-disk is ARGB. Cross-checked
  against xenia-rs decode_k8888_tiled, the Canary-validated M1/M2 path,
  which nets the same [A,R,G,B]->[R,G,B,A]. sylpheed-formats is correct.

Promotes the k8888 entry HYPOTHESIS -> CONFIRMED; INDEX updated. XPR2
skybox tiling + viewer display-gamma remain separate open items.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/re/INDEX.md

@@ -14,7 +14,7 @@ Promote to a prose `structures/…md` file when a format needs behavioural notes
 | name-hash (TOC keys) | ✅ | `sylpheed-formats/src/hash.rs` | Barrett-reduction hash; recovers original paths |
 | IDXD object/table | ✅ | `sylpheed-formats/src/idxd.rs` | self-describing; ship/weapon stats verified vs known values |
 | XPR2 texture + cubemap | 🟡 | `sylpheed-formats/src/texture.rs` | de-tile + A8R8G8B8; **colours unverified** (dynamic item) |
-| T8aD 2D texture | 🟡 | `sylpheed-formats/src/t8ad.rs` | ~85% decode; **colours unverified**; ~15% variants deferred |
+| T8aD 2D texture | 🟡 | `sylpheed-formats/src/t8ad.rs` | ~85% decode; **colours ✅ CONFIRMED** ([k8888](structures/texture-color-k8888.md)); ~15% variants deferred |
 | RATC bundle | 🟡 | `sylpheed-formats/src/ratc.rs` | child listing confirmed; one level deep |
 | LSTA sprite list | 🟡 | `sylpheed-formats/src/lsta.rs` | inline T8aD frames |
 | IXUD subtitle | 🟡 | `sylpheed-formats/src/ixud.rs` | timed cues; **movie↔track link unknown** (dynamic item) |