docs(skill): the surface table, rig anchors, and asserting the consequence

The skill is this project's own instructions, and three commits made it wrong:
it described a runtime that re-guessed what every surface was, a lab that only
tuned weapon holds, and a suite that could not see any of the last five defects.
It also still listed a seventh character that is not in skins.json.

Records what is new — the surface table and how it is decided, the layered
tuning files and why a fixed rotation on the weapon mount is the wrong answer,
the rig lab — and what the work taught:

  * assert the CONSEQUENCE. Asking a model which clip it is playing reads a
    variable it set on itself, and says "Idle" just as happily when nothing is
    ticking. Asking whether an anchor saved says nothing about whether the gun
    moved.
  * four of the last five real defects came from looking at a PNG. Every one of
    them passed every assertion.
  * the line-work rule now exists twice, at build time and as the runtime
    fallback, and they must be changed together or a model with a surface table
    starts rendering differently from one without.

Also records momo's broken idle and the stray Icosphere under known-unsolved,
with what is worth suspecting first in each case, since neither is fixed.

Co-Authored-By: Claude Opus 5 <[email protected]>

.claude/skills/character-pipeline/references/separation.md

@@ -19,6 +19,7 @@ the convention):
 | Cloth/hair bones carry NO animation keys; physics owns them | `export_optimize_animation_keep_anim_armature=False` |
 | Physics colliders are a small set of capsules: thighs, shins, and a big one at the waist acting as a lid | 5 capsules, measured from the mesh (`_leg_colliders`) |
 | Neighbouring skirt panels are linked sideways | 278 cross-panel distance links from shared vertices |
+| Each surface is TAGGED with what it is, so shading can differ per class | `tools/surface_map.py` writes it; `SkinSurfaces` reads it |
 | Cel shading with a ramp, plus a separate outline pass | `LevelMaterials.apply_toon_recursive` + `apply_character_look` |
 
 Where we differ: their collider capsules and cloth parameters are hand-authored
@@ -26,6 +27,38 @@ per character by a technical artist. We MEASURE them from the model's own
 geometry at build time, because there is no artist in this loop. That is the
 whole reason `<model>.rig.json` exists.
 
+Where there IS an artist in the loop, there is now somewhere to put the answer:
+`debug/rig_lab.tscn` and the layered files behind it (see the SKILL). Measuring
+is the default and hand-authoring is the override, rather than the other way
+round.
+
+## Separation is only half of it — the parts have to be NAMED
+
+Keeping the meshes apart is structural. Knowing which is which is what lets
+anything act on the difference, and until the surface table existed nothing did:
+every surface of every character took one set of shading numbers, calibrated on
+skin, because there was no way to ask whether a surface was hair.
+
+The table lives in the sidecar as `surfaces`, keyed on the MATERIAL name — mesh
+node names are `Object_7` through `Object_32` on every character in this game and
+carry no meaning, while material names survive the glTF round trip intact and are
+what the artist actually chose. It is decided three ways, in descending order of
+how much it trusts them:
+
+1. **the material name.** On VRoid exports it is formal —
+   `N00_000_00_Body_00_SKIN_Instance` carries its own class infix, and every
+   VRoid character here uses SKIN / FACE / EYE / HAIR / CLOTH.
+2. **the weights.** Decisive when the name says nothing: a surface pulled by the
+   skirt chain is a skirt whatever it is called. The threshold is deliberately
+   low (5%), because VRoid welds the whole cap of the hair to the head bone and
+   springs only the strands — kiyoko's hair mesh is 85% head, and a majority rule
+   would call it skin.
+3. **the material flags.** These catch line-work, which is the one class that is
+   not a surface of the character at all.
+
+It is built from the same chains the spring solver uses, so the two can never
+disagree about which bones are a skirt.
+
 ## Why the separation is load-bearing
 
 **Materials.** The body wants skin shading, hair wants an anisotropic-ish ramp