[Feature] Add root motion extraction and application

[miogds]

Summary

Next link in the animation chain after inertialized transitions (#1125): root motion. A locomotion clip authored with a traveling root previously dragged the mesh away from the entity transform and snapped it back on every loop. With root motion enabled — opt-in per entity, default off, zero cost when disabled — the root joint's horizontal translation and yaw deltas are extracted from the raw sampled pose each frame and applied to the entity transform in character space (translateBy / rotateTo), while the pose root is grounded: horizontal travel zeroed, yaw removed via swing–twist decomposition. Vertical motion, pitch, and roll stay in the pose — a crouch still lowers the character, a stagger still leans it.

API

setRootMotionEnabled(entityId: zombie, enabled: true)                              // default root = first parentless joint
setRootMotionEnabled(entityId: zombie, enabled: true, rootJointPath: "root/hips")  // rig-specific override
isRootMotionEnabled(entityId: zombie)

• No loop snap: the clip's per-loop root displacement and yaw are precomputed as CompiledAnimationClip root metadata, and the wrapped-time jump is corrected with them.
• Anchored to the gameplay handle: hierarchical assets (setEntityMeshAsync) carry the AnimationComponent on a skinned child while the game steers the asset root. Deltas are applied to the entity the public API was called on, so nothing drifts inside the asset. Flat entities behave exactly as before.
• Composes with transitions: root motion runs on the raw sampled pose before transition offsets, and the transition capture grounds the incoming clip's samples — inertialized transitions blend grounded poses and never teleport the character. changeAnimation re-baselines extraction, so a clip switch contributes no spurious delta.
• Defends against LocalTransformComponent's zero-quaternion default rotation (which rotates every vector to zero) by treating it as identity.

How-to guide: docs/API/UsingRootMotion.md. Single commit cherry-picked onto current develop (5f61789).

Testing

AnimationRootMotionTests (11 tests, all passing):

• Off by default; enable/disable round-trip
• Entity accumulates clip displacement across loops (exact expected travel), no snap at loop wrap
• Pose root grounded (horizontal zeroed, vertical kept); yaw accumulates on the entity and is stripped from the pose
• Transition mid-walk blends grounded poses; clip switch does not teleport the entity
• Hierarchical asset: APIs called on the parent root move the parent, child never drifts
• Root joint path override

Inertialization, compiled-sampler, and policy suites still pass on this base. SwiftFormat lint clean.