Add Phase 9 motion acceptance coverage

[snkmcb]

Summary

• add deterministic Phase 9 skeletal E2E coverage from VMD-derived leg IK through motionUsd and motionRetarget
• verify both a PMX-derived UsdSkelSkeleton target and an MMD-free generic target path
• pin motionUsd v0.5.0 for acceptance testing and complete mmdSkeletonAdapter's external motionCore closure
• record the successful synthetic and local-asset acceptance runs and update Phase 9 status documentation

Validation

• ost library test libs/mmdMotionAdapter --target cy2026 --profile usd — 3/3 passed
• ost library test libs/mmdSkeletonAdapter --target cy2026 --profile usd — 2/2 passed
• ost plugin test --workspace --target cy2026 --profile usd — passed
• python scripts/check_docs.py — passed
• git diff --check — passed

Review follow-up

• clarified that MOT-O10 still needs a measured A-pose-to-level-arm comparison
• removed reliance on transitive standard-library includes in the acceptance test