Define the public live motion stream boundary

[snkmcb]

Summary

• Resolve MC-O5 by naming the existing LiveCaptureSource::Push and IMotionSource::Sample APIs as the public live motion stream boundary.
• Document actor-scoped routing, stream provenance, and per-sample timestamp and sequence preservation.
• Update the motion contract, capability matrix, roadmap, README, and changelog to reflect the decision.

Companion connector PR: animu-sphere/motion-connectors#20

Type of change

• Bug fix
• Feature or behavior change
• Documentation or test change
• Build or maintenance change

Validation

• scripts/check_docs.py --selftest passed.
• scripts/check_docs.py passed (39 Markdown files).
• git diff --check passed.

Checklist

• The change is focused and ready for review.
• I added or updated tests when behavior changed. No motion library behavior changed; the companion connector PR adds the handoff test.
• I updated the owning documentation when a public fact or capability changed.
• I did not add a motion capture, clip, avatar model or screenshot without redistribution permission.
• This pull request does not contain a security report.