Avatar customization: skin tone, head swap, and hair now actually apply

Three dead controls on the customization screen:

- Skin tone slider did nothing: neither the visualizer nor AvatarSystem
  ever read loadout.skin_tone. Added AvatarLoader.apply_skin_tone() -- it
  retints the base body's skin surfaces (matched by the placeholder skin
  albedo, i.e. head + arms) -- and both build paths call it. Slider now
  maps across a deep-to-pale skin range.

- "Cyber Head" did nothing: its catalog entry was type "body_part", which
  load_attachment() rejects, and the placeholder body is one mesh so there
  was nothing to hide either. Retyped it as an attachment -> it mounts on
  the Head bone as a cyber visor.

- Mohawk / ponytail sat inside the head: the Head and HeadTop bones were
  well below the visual head (bone at 1.58, head crown at ~2.03). Moved
  Head to the head's centre and HeadTop just above the crown (mesh is
  rigidly bound, so it doesn't move), and retuned the hair/visor/glasses
  meshes and offsets to match.

Verified: avatar smoke 14/14 (source + pack), fresh-clone, 2p multiplayer,
and drove the real customization screen in rendered screenshots (equip
head+mohawk+mid skin, then ponytail+glasses+pale skin).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

assets/avatars/catalog.json

@@ -8,7 +8,7 @@
       "display_name": "Human Male Base"
     },
     "head_cyber_01": {
-      "type": "body_part",
+      "type": "attachment",
       "slot": "head",
       "path": "res://assets/avatars/head_cyber_01.glb",
       "version": "1.0.0",