Parse tfrag topology: vertex info, strips, indices and triangles

Turns tfrag positions into drawable geometry. Every unpack in a patch's
VIF lists is now classified by format *and* destination VU address, using
the VU header at the head of the common list. That table is what tells
the strips from the indices from the parent arrays: they are all V4_8
and only the address distinguishes them.

Strip indices address the vertex-info array, not the position array.
Several vertex-info entries can share a position, so an index resolves
through vertex_info[i].vertex / 2 to reach a position. Vertex-info also
carries UVs and the LOD parent link.

A strip run of n vertices is a GS triangle strip of n - 2 triangles with
alternating winding. The vertex count is biased by -128 when negative,
and the first record of a list is always biased even when positive --
real patches open with -116 or -124, never a small positive number.

tri_count lives in the patch header and cannot be derived from the lists
without walking them, so reproducing it is a strong check: all 27 levels,
18,102 patches, 809,879 triangles, every patch matching its header, every
triangle index inside its LOD's vertex-info range.

Four wrong turns got there, each of which failed quietly rather than
loudly: reading the element size from the immediate instead of the
command, 2-byte payload padding instead of 4, ignoring the inline
payload of STROW/STCOL/STMASK/MPG/DIRECT, and banking strips by arrival
order instead of by LOD. Every one produced parseable, plausible geometry.
Only the header's own triangle count caught them.

Still open: ad-gif texture bindings, per-vertex RGBA and light records,
and the LOD switch and morph rules.

docs/plan/level-assets.md

@@ -313,7 +313,52 @@ mismatch means the walk desynchronised, so `tfrag_positions` returns `None`
 rather than partial geometry. Across all 27 levels: 18,102 patches,
 821,439 vertices, zero mismatches.
 
+### Topology: strips, indices and triangles
+
+Also settled. `parse_tfrag` walks each list and classifies every unpack by
+format *and* destination VU address, using the VU header at the head of the
+common list (5 V4_16 vectors, unsigned, to address 0). That table is what
+separates the strips from the indices from the parent arrays — they are all
+V4_8, and only the address tells them apart.
+
+| `(vn, vl)` | Meaning |
+|---|---|
+| (3, 1), unsigned, addr 0 | the VU header itself |
+| (3, 0) | ad-gifs, 5 quadwords each |
+| (2, 1) | positions, V3_16 |
+| (3, 1) | vertex info: UV, LOD parent, own position |
+| (3, 2) | strips (signed) and indices (unsigned) |
+
+**Strip indices address the vertex-info array, not the position array.** Several
+vertex-info entries can share a position, so a strip index resolves through
+`vertex_info[i].vertex / 2` to reach a position.
+
+A strip record is four bytes: a vertex count, a packet flag and an ad-gif
+offset. A negative count is biased by −128, and *the first record of a list is
+always biased* even when its count is positive — a real patch opens with −116 or
+−124, never a small positive number. A zero count ends the list. A run of `n`
+vertices is a GS triangle strip of `n − 2` triangles with alternating winding.
+
+**Bank the LODs by index, not by arrival.** The lists are walked in transfer
+order but the strip and index arrays have to be stored by *which LOD they
+belong to*. Appending them as they are found fills the arrays LOD-2-first,
+which looks fine until the triangle counts come out low.
+
+### Verification
+
+`tri_count` is stored in the patch header and cannot be derived from the lists
+without walking them, so reproducing it is a strong check. All 27 levels:
+**18,102 patches, 809,879 triangles, every patch matching its header**, with
+every triangle index inside that LOD's vertex-info range and every ad-gif index
+inside the ad-gif array.
+
+Getting there took four wrong turns, each of which failed quietly: reading the
+element size from the immediate, 2-byte payload padding, ignoring non-unpack
+payloads, and banking strips by arrival order. Every one produced parseable,
+plausible geometry; only the header's own triangle count caught them.
+
 ### Still open
 
-Strips and triangle indices, the RGBA vertex-info arrays, texture binding, and
-the LOD/morph/cull rules in `TfragProc`. Positions alone are not drawable.
+The ad-gif texture bindings (which texture each primitive samples), the RGBA
+per-vertex colours and light records, and the LOD switch and morph rules in
+`TfragProc`. Positions and topology are drawable but not yet textured.