XY-295: move scroll capture capability ownership into app host

[yvette-carlisle]

Summary

• move scroll-capture capability acquisition behind an explicit host adapter owned by the app host
• keep stitching and overlap-proof semantics in rsnap-overlay while adding structured host busy/unavailable handling
• add regression coverage for silent permission rejection and busy frame-request backoff, and update host/core docs

Testing

• cargo make lint
• cargo make test
• cargo make replay-scroll-capture-self-check
• cargo make smoke-self-check-macos

Notes

• cargo make replay-scroll-capture still requires local traces in ~/Library/Application Support/ink.hack.rsnap/scroll-capture-traces on this machine
• manual live scroll-capture validation was not run in this session