fix: make the character pipeline work end-to-end in Blender 5.1 + bundle CC0 animation library

Verified the full chain on a real Sketchfab model (Miku UID -> rigged ->
13 clips -> loads and animates in-engine, 24/24 smoke checks pass).

Fixes found while running it for real:
- sketchfab_import: read .sketchfab_token as UTF-16/BOM too (PowerShell's
  `echo >` writes UTF-16LE, which crashed the UTF-8 reader)
- autorig: rig to the animation library's OWN skeleton when _library.glb
  is present (zero retargeting); robust binding — fall back to rigid
  nearest-bone weights when Blender bone-heat fails on complex meshes
  (Miku's joined hair/clothing gave 0 weighted verts); export as
  parent_type=ARMATURE with no modifier (Blender 5.x only emits a glTF
  skin in that exact form — a lingering Armature modifier gave skins:0);
  export_apply=False (applying modifiers baked away the skin); flatten
  the Sketchfab empty hierarchy before binding
- merge_animations: support a single multi-clip library GLB via
  LIBRARY_CLIP_MAP; version-safe fcurve access for Blender 4.4+/5.x
  slotted actions (Action.fcurves was removed)

Assets/tooling:
- bundle assets/characters/animations/_library.glb — CC0 Quaternius
  Universal Animation Library (13 game clips), + .gdignore so Godot
  skips the pipeline-input folder
- requirements.txt (stdlib-only; documents Blender-python separation)
- pipeline docs updated for the bundled library + swap instructions

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

assets/characters/skins/miku_test.license.json

@@ -0,0 +1,9 @@
+{
+  "name": "Hatsune Miku",
+  "uid": "34f3e7daa4c64c8a8000ae7f90b01ceb",
+  "author": "Tigerar1",
+  "author_url": "https://sketchfab.com/allanromanreyes",
+  "license": "Free Standard",
+  "license_slug": "free-st",
+  "source_url": "https://sketchfab.com/3d-models/hatsune-miku-34f3e7daa4c64c8a8000ae7f90b01ceb"
+}