Bend limbs along a single axis with a Bézier spine

- core: Bezier (any degree, De Casteljau), LimbBend (cubic spine through
  the rigid upper/joint/lower shape, one ring per texture row spaced by
  arc length, rigid rings so pieces share vertices) and QuadSplitter
  (cuts faces at the rings, interpolating UVs).
- fabric: ModelPartMixin draws parts that have a bend with
  BentCubeRenderer, which deforms vanilla's own cubes. Overlays and armor
  use the limb's spine so they stay attached. Bends are reset every
  setupAnim because models are shared between entities.
- testmod: flap and squat animations, and a client gametest that
  screenshots each animation from the front and side
  (runClientGameTest). The tester runs with the full Fabric API because
  single modules don't bring their version-specific dependencies.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/testmod/resources/fabric.mod.json

@@ -7,7 +7,8 @@
   "license": "Apache-2.0",
   "environment": "client",
   "entrypoints": {
-    "client": ["org.saturnclient.emotes.testmod.EmotesTestmod"]
+    "client": ["org.saturnclient.emotes.testmod.EmotesTestmod"],
+    "fabric-client-gametest": ["org.saturnclient.emotes.testmod.EmotesGameTest"]
   },
   "depends": {
     "saturn_emotes": "*",