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.
