Fix critical bugs in animation system: slerp, keyframe sampling, asset loading

[Copilot]

Animation system had multiple bugs rendering it non-functional: broken quaternion interpolation, uninitialized keyframe sampling, incorrect asset deserialization, and unimplemented sampling logic.

Quaternion slerp fix (AnimationInterpolation.cpp)

Return value had wrong component mapping. Constructor is Quaternion(w, x, y, z) but brace-init was {x, w, z, w}:

// Before (broken)
return { k1*vk.x + k2*vk1.x, k1*vk.w + k2*vk1.w, k1*vk.z + k2*vk1.z, k1*vk.w + k2*vk1.w };

// After
return { k1*vk.w + k2*vk1.w, k1*vk.x + k2*vk1.x, k1*vk.y + k2*vk1.y, k1*vk.z + k2*vk1.z };

Keyframe sampling fix (AnimationInterpolation.h)

• AnimSampleChannel never populated vk1/vk2 from channel data and t was always 0 — now properly extracts surrounding keyframes and computes interpolation parameter
• Handles edge cases: timepoint before first key or after last key
• Fixed call to undefined SphericalLinear() → AnimSphericalLinear()
• Fixed (1-t) * vk → vk * (1-t) — Vector3 has no free operator*(float, Vector3)

Asset loading fix (AnimationAsset.cpp)

Copy-paste error: rotation time data was loaded into channel.position.time instead of channel.rotation.time.

Implement AnimationNodeChannel::Sample

Was an empty stub. Now delegates to AnimSampleChannel for each non-empty channel (position, scale, rotation) and writes results to a new AnimNodeSampleResult struct.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.