M6: MotionForge boundary integration

[TristinWen]

Goal

Define a clean ShapeForge ↔ MotionForge boundary.

ShapeForge owns rest pose, stable nodes, semantic rig roles, constraints, and transform targets. MotionForge owns engine-neutral motion intent/IR, clips, tracks, keyframes, curves, composition, and serialization. Native adapters own playback, blending, IK, retargeting, and optimization.

[TristinWen]

Implemented in 56cd954: cached semantic role bindings, authored rest poses, joint constraints, writable transform targets, constraint clamping, rest reset, tests, and an explicit ownership boundary. Motion data and playback remain outside ShapeForge as required. Live EditMode execution remains pending.