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]>

requirements.txt

@@ -0,0 +1,11 @@
+# Python dependencies for tools/ (sketchfab_import.py, pipeline.py).
+#
+# Currently empty: these scripts only use the standard library
+# (urllib, json, zipfile, argparse, subprocess). No third-party
+# packages to install — this file exists so `pip install -r
+# requirements.txt` works out of the box in a fresh venv.
+#
+# NOT covered here: tools/autorig.py and tools/merge_animations.py run
+# inside Blender's own bundled Python via `blender --background
+# --python ...`, not this venv. Their `bpy` / `mathutils` imports come
+# from Blender itself and cannot be pip-installed.