feat(characters): import the Quaternius mannequin as a selectable skin

A third playable character, built with the pipeline skill from a source
that was already in the repo: the animation library ships a rigged
Mannequin mesh on the exact 53-joint reference skeleton, CC0, so it needed
no download and retargets perfectly. 18 clips, 0.3% cross-leg bleed, 7% of
verts at four influences — a clean authored-weight import. Licence
recorded in mannequin.license.json as the other skins do.

It has no cloth chains, correctly: it is a mannequin and has neither hair
nor clothes.

Importing it turned up two real bugs, both of which would have hit any
flat-coloured or single-piece model:

- LevelMaterials.apply_character_look treated ANY untextured surface on a
  character as the model's own outline shell and hid it, so the mannequin
  rendered as a solid black silhouette — its body and joint materials are
  untextured flat colours, not ink. _is_line_work() now asks whether the
  surface is named eyes*, is drawn front-face-culled (the inverted-hull
  setup), or is near-black. Taila and Miku are unaffected: their materials
  are textured and never reach that branch. Verified by render.

- verify_character.py failed the build for having one mesh. That check
  cannot tell "the pipeline joined them" from "the artist authored one
  mesh" — Quaternius' mannequin is one piece on purpose. It is advisory
  now; the join path's two unambiguous signatures, cross-leg bleed and the
  4-influences-everywhere spread, are still hard checks.

Also restored Miku's description, which the re-import had blanked.

3 GLB skins selectable (6 with the built-in colour skins). Smoke 0
failures, 11/11 movement tests, cloth idle 0.024-0.078 deg/frame.

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

assets/characters/skins/skins.json

@@ -3,7 +3,7 @@
     {
       "id": "miku",
       "name": "Miku",
-      "description": "",
+      "description": "Hatsune Miku \u2014 Virtual Idol",
       "model": "res://assets/characters/skins/miku.glb",
       "unlocked": true
     },
@@ -13,6 +13,13 @@
       "description": "",
       "model": "res://assets/characters/skins/taila.glb",
       "unlocked": true
+    },
+    {
+      "id": "mannequin",
+      "name": "Mannequin",
+      "description": "Quaternius reference mannequin (CC0)",
+      "model": "res://assets/characters/skins/mannequin.glb",
+      "unlocked": true
     }
   ]
 }