Draw tfrag per-vertex colour, unlit, and record why the frame is dark

The tfrag RGBA array now modulates the texture. It is indexed by
*position*, not by vertex-info: several vertex-info entries can share a
position, and the array is sized vert_count, which is a position count.

Colour follows the same rule as the vertex itself. Morphing blends the
position only -- the game keeps the high-detail colour -- while collapse
replaces the vertex outright, colour included. Getting that the other way
round would tint distant terrain.

The material is unlit, and that is a decision rather than a shortcut.
Level 0's colours really are dark (patch 0 runs (25,26,26) to
(108,144,141)) and the game's lighting is what brightens them, so what is
on screen is the true albedo.

Worth recording: the obvious move -- add a directional light, or crank
ambient brightness -- does nothing at all. With no normals the PBR path
is degenerate, so the brightness change has no effect, and a host light
would have hidden the gap rather than shown it. The colour data was
always reaching the mesh; what is missing is lighting, not a material
flag.

Written up in plan/level-assets.md, including what LightTfrags actually
needs: the light bank from the level's gameplay extent, the 256-entry
normal table at BOOT_NORMAL_TABLE_VADDR (0x165500) which every level
overlay carries, and the already-parsed azimuth/elevation light records.

docs/roadmap.md

@@ -31,8 +31,16 @@ early, while they are still cheap to retire.
       passed: the vertex count matched, the triangle count matched, only the
       *place* was wrong. Fixed by taking the STROW immediately preceding the
       first position unpack rather than the last one.
-- [ ] Next: the per-vertex light records, which is why the first frame is flat
-      and dark. Then the GS register set and the guard-band clip test.
+- [x] **Per-vertex colour.** The tfrag's RGBA array now modulates the texture,
+      drawn unlit so the frame shows real albedo rather than a faked lighting
+      model. Verified against the stored values (level 0 patch 0 runs
+      `(25,26,26)` to `(108,144,141)`), which are genuinely dark.
+- [ ] **tfrag lighting.** Needs the level light bank from `gameplay`, the
+      256-entry normal table from the overlay ELF at `0x165500`, and the
+      already-parsed azimuth/elevation light records. The pass itself is
+      `base + A*max(dA, dA*wA) + B*max(dB, dB*wB)` and has to run in the PS2
+      float model. Detail in `plan/level-assets.md`.
+- [ ] Then the GS register set and the guard-band clip test.
 
 The disc being the exact build the reference projects pin is the single
 biggest favourable fact so far: no disc conversion, no regional timing work,