feat: initial implementation with build_rig + build_animation

Validates and cooks rig + animation data into internal representations:
- build_rig: parses bones (with parent-resolution to object refs) and
  tracks (with bone-uniqueness validation). Converts rest angles
  from JSON degrees to radians.
- build_animation: validates each keyframe references only bones in
  the animation's declared track. Converts keyframe angles from
  degrees to radians.

Remaining functions (sample_animation, spawn, update, look-at,
procedural, render, lifecycle) follow in subsequent commits.

manifest.lib

@@ -0,0 +1,6 @@
+{
+    "id": "lib-core.puppet",
+    "version": "0.1.0",
+    "api_min": "0.1",
+    "deps": []
+}