Fix scroll capture stitching validation

[yvette-carlisle]

Summary

• rework native scroll-capture observation to carry motion hints into worker-pairwise registration
• fail closed when selected regions include static informative side content
• add deterministic positive/negative native scroll-capture smoke coverage and export continuity validation

Verification

• cargo make check
• scripts/perf/local.sh
• scripts/smoke/native-hud-follow-macos.sh
• scripts/smoke/native-visual-contract-macos.sh
• SCROLL_BACKGROUND_MODE=codex_like SCROLL_COUNT=4 SCROLL_INTERVAL_MS=600 POST_SCROLL_SETTLE_S=1.0 scripts/smoke/native-scroll-capture-macos.sh
• SCROLL_BACKGROUND_MODE=full_document SCROLL_BACKGROUND_PROOF_STRIPE=1 VALIDATE_SCROLL_EXPORT_CONTINUITY=1 SCROLL_COUNT=8 SCROLL_INTERVAL_MS=800 POST_SCROLL_SETTLE_S=3.0 scripts/smoke/native-scroll-capture-macos.sh