Samples an animation at time t. Wraps t for loops. Finds the
keyframe segment containing t and linearly interpolates per-bone
angles. Returns frame with angles in degrees (intuitive for tests
and callers; internal storage uses radians).
Edge cases handled:
- t = 0 returns first-keyframe values
- t = duration returns last-keyframe values (no overshoot)
- t > duration with loop=true wraps via modulo
- Keyframes with disjoint bone-sets: each bone interpolated where
available, snapped where only one side has it.
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.
