fix(pipeline): check that a character is CORRECT, not merely well-formed

Seven characters shipped "All checks passed" and four were visibly broken
in game — lying on their backs at seven times scale, facing backwards, or
holding a gun that floated near their chest. Nothing in the suite was
wrong; it just never asked the questions that mattered. That distinction
is the whole lesson, and it is now written down in the skill as
references/failure-modes.md, generalised per failure.

Two root causes are measured and certain:

- flatten_and_scale() normalises the bounding box along Blender Z because
  Z is up. For a model that arrives lying along Y that measures the
  character's THICKNESS, so it scales by ~7 and leaves them on their back.
  One assumption, both symptoms. The trap is that the normalised number
  always comes out right — the exporter maps Blender Z to glTF Y, so "is
  the height 1.75" passes on a character who is 7.5 m tall lying down.
  All three casualties are VRM files that went through a Blender
  round-trip and came back with a baked axis rotation.

- SkinnedPlayerModel.set_weapon() finds the hand with three hardcoded
  spellings, which match none of the four non-Rigify rigs — their hands
  resolve perfectly in the sidecar as "Right wrist" and "J_Bip_R_Hand".
  When it misses, the weapon is parented to the model root at a fixed
  chest offset, so it is not attached to the character at all. Same class
  of bug as the leg check that name-matched thigh/shin. rig_map.py exists
  so nothing downstream has to guess a bone name; only some consumers read
  the roles it publishes.

Three new hard checks, none needing more than the vertices and the
sidecar:

  character stands up in world space   — against WORLD up, not against the
      model's own proportions. "Is the spine the longest axis?" catches
      nothing: a model rotated as a whole is internally consistent and
      passes it comfortably.
  character is a plausible size / height
  every role the runtime needs is resolved

They separate the four good characters from the three broken ones on the
first run. Also fixed the posture measurement to read vertices rather than
object.bound_box, which is cached and still stale right after an import —
it reported a 1.75 m character as 1.18 m tall.

Recorded but not yet fixed: kiyoko faces backwards (facing is inferred by
two independent mechanisms and verified by neither), miku's grown hair
chains stretch under animation (generated chains are never validated
against the geometry they drive), and the mannequin's rifle hold does not
convince despite resolving correctly.

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

.claude/skills/character-pipeline/SKILL.md

@@ -81,6 +81,9 @@ Blender is required (`BLENDER_PATH`, or auto-found under
 Load the reference that matches what you are doing. They are short and each one
 is a list of things that cost a debugging cycle to learn.
 
+- **`references/failure-modes.md`** — **read this first.** Seven characters
+  shipped "All checks passed" and four were visibly broken. What each failure
+  was, why the suite missed it, and the rule that generalises it to any model.
 - **`references/separation.md`** — body vs. garments vs. hair: what must stay
   separate, how cloth chains are detected and classed, why cloth is never
   skinned to a leg, and the ZZZ-convention mapping.
@@ -112,6 +115,16 @@ is a list of things that cost a debugging cycle to learn.
    deletes triangles. It exists only to undo auto-rigging. It is gated on
    `weights_authored`, which is MEASURED, not assumed.
 5. **Measure from inside the modifier pass.** See `references/verification.md`.
+6. **Never assume an axis.** Up, forward and scale are all measurable from the
+   skeleton. Assuming +Z is up scaled three characters 7x and left them on their
+   backs — and the height check passed on every one of them, because the number
+   being normalised always comes out right whether or not it was the right
+   number. See `references/failure-modes.md`.
+7. **Never look a bone up by name.** `tools/rig_map.py` resolves roles and writes
+   them to the sidecar so nothing downstream has to guess. Any hardcoded spelling
+   — `_find_bone(["RightHand", ...])`, a `thigh`/`shin` substring test — is a rig
+   this project has not met yet. Four characters could not hold a gun because of
+   exactly one such lookup.
 
 ## After importing
 
@@ -132,6 +145,7 @@ What "good" looks like on Taila, for calibration:
 | Leg inside cloth, run/slide/dash | ~95 mm *(current, unsolved)* | — |
 | Solver cost | ~2.6 ms/character | 10 ms |
 | Stride vs. travel direction | < 10° (except a capped sidestep) | 90° |
+| Bind-pose AABB | tall on the hips→head axis, others < 2.5 m | tallest axis is depth |
 
 ## Characters currently shipping