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/mannequin.rig.json

@@ -0,0 +1,131 @@
+{
+  "roles": {
+    "hips": "DEF-hips",
+    "head": "DEF-head",
+    "neck": "DEF-neck",
+    "spine": [
+      "DEF-spine.001",
+      "DEF-spine.002",
+      "DEF-spine.003",
+      "DEF-neck",
+      "DEF-head"
+    ],
+    "upper_arm.R": "DEF-upper_arm.R",
+    "upper_arm.L": "DEF-upper_arm.L",
+    "forearm.R": "DEF-forearm.R",
+    "toe.R": "DEF-toe.R",
+    "hand.L": "DEF-hand.L",
+    "thigh.R": "DEF-thigh.R",
+    "foot.R": "DEF-foot.R",
+    "shin.L": "DEF-shin.L",
+    "shin.R": "DEF-shin.R",
+    "thigh.L": "DEF-thigh.L",
+    "toe.L": "DEF-toe.L",
+    "shoulder.L": "DEF-shoulder.L",
+    "shoulder.R": "DEF-shoulder.R",
+    "hand.R": "DEF-hand.R",
+    "foot.L": "DEF-foot.L",
+    "forearm.L": "DEF-forearm.L"
+  },
+  "chains": [],
+  "twist": [],
+  "colliders": [
+    {
+      "bone": "DEF-hips",
+      "child": "DEF-spine.001",
+      "from": 0.0,
+      "radius_head": 0.1381,
+      "radius_tail": 0.1381,
+      "radius": 0.1381,
+      "lid": true
+    },
+    {
+      "bone": "DEF-thigh.L",
+      "child": "DEF-shin.L",
+      "from": 0.1,
+      "radius_head": 0.1037,
+      "radius_tail": 0.0733,
+      "radius": 0.0733
+    },
+    {
+      "bone": "DEF-thigh.R",
+      "child": "DEF-shin.R",
+      "from": 0.1,
+      "radius_head": 0.1037,
+      "radius_tail": 0.0733,
+      "radius": 0.0733
+    },
+    {
+      "bone": "DEF-shin.L",
+      "child": "DEF-foot.L",
+      "from": 0.1,
+      "radius_head": 0.0927,
+      "radius_tail": 0.0516,
+      "radius": 0.0516
+    },
+    {
+      "bone": "DEF-shin.R",
+      "child": "DEF-foot.R",
+      "from": 0.1,
+      "radius_head": 0.0927,
+      "radius_tail": 0.0516,
+      "radius": 0.0516
+    }
+  ],
+  "weights_authored": true,
+  "driven_bones": [
+    "DEF-f_index.01.L",
+    "DEF-f_index.01.R",
+    "DEF-f_index.02.L",
+    "DEF-f_index.02.R",
+    "DEF-f_index.03.L",
+    "DEF-f_index.03.R",
+    "DEF-f_middle.01.L",
+    "DEF-f_middle.01.R",
+    "DEF-f_middle.02.L",
+    "DEF-f_middle.02.R",
+    "DEF-f_middle.03.L",
+    "DEF-f_middle.03.R",
+    "DEF-f_pinky.01.L",
+    "DEF-f_pinky.01.R",
+    "DEF-f_pinky.02.L",
+    "DEF-f_pinky.02.R",
+    "DEF-f_pinky.03.L",
+    "DEF-f_pinky.03.R",
+    "DEF-f_ring.01.L",
+    "DEF-f_ring.01.R",
+    "DEF-f_ring.02.L",
+    "DEF-f_ring.02.R",
+    "DEF-f_ring.03.L",
+    "DEF-f_ring.03.R",
+    "DEF-foot.L",
+    "DEF-foot.R",
+    "DEF-forearm.L",
+    "DEF-forearm.R",
+    "DEF-hand.L",
+    "DEF-hand.R",
+    "DEF-head",
+    "DEF-hips",
+    "DEF-neck",
+    "DEF-shin.L",
+    "DEF-shin.R",
+    "DEF-shoulder.L",
+    "DEF-shoulder.R",
+    "DEF-spine.001",
+    "DEF-spine.002",
+    "DEF-spine.003",
+    "DEF-thigh.L",
+    "DEF-thigh.R",
+    "DEF-thumb.01.L",
+    "DEF-thumb.01.R",
+    "DEF-thumb.02.L",
+    "DEF-thumb.02.R",
+    "DEF-thumb.03.L",
+    "DEF-thumb.03.R",
+    "DEF-toe.L",
+    "DEF-toe.R",
+    "DEF-upper_arm.L",
+    "DEF-upper_arm.R",
+    "root"
+  ]
+}