feat(client): character creator overhaul — manifest, segments, eye color

Major changes from runtime testing and iteration:
- Asset manifest (manifest.json) controls all available content —
  replaces filesystem scanning, solves DirAccess/PCK export issue
- Dynamic tabs: only show tabs with manifest content
- Body segmentation: face-based exclusive assignment, hips split from
  torso, torso_upper independent (not a variant)
- Eye color with iris-only mask from T_Eye_Split.png green channel
- Eyebrows tinted with hair color (from body segment, not separate GLB)
- Hair/clothing loaded as skinned meshes on shared skeleton
- Segment hiding disabled for clothing (solidify handles coverage)
- Gender-aware randomizer (no facial hair on female/teen)
- Clothing slots hidden when empty in manifest
- UI: color docks docked to bottom on all tabs, padding, flow layout
- Debug tab with per-segment visibility toggles
- Screenshot automation with test config JSON

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/assets/characters/manifest.json

@@ -0,0 +1,13 @@
+{
+  "body_types": ["average_m", "average_f", "muscular_m", "muscular_f", "teen_m", "teen_f"],
+  "heads": [],
+  "hair": ["bob", "buns", "buzzed", "long", "ponytail", "bald"],
+  "facial_hair": ["beard", "moustache", "mutton_chops"],
+  "eyebrows": [],
+  "clothing": {
+    "peasant_tunic": {"slot": "torso"},
+    "peasant_pants": {"slot": "legs"},
+    "peasant_shoes": {"slot": "feet"}
+  },
+  "accessories": []
+}