Optimize scroll capture responsiveness

[yvette-carlisle]

Summary

• keep scroll capture stitching alive for near-viewport/right-side wheel input and stale live stream cases
• make toolbar liquid-glass sampling live-first with a lightweight fallback and add smoke/spec coverage
• add lightweight scroll preview export and harden native-host staging so local runs do not reuse stale builds

Verification

• RSNAP_NATIVE_HOST_FORCE_REBUILD=1 RSNAP_NATIVE_HOST_SWIFT_CLEAN=1 scripts/build_and_run.sh --verify
• SCROLL_START_METHOD=keyboard SCROLL_POINT_MODE=right_outside_selection MIN_SCROLL_COMMITS=4 MIN_EXPORT_GROWTH_PX=300 scripts/smoke/native-scroll-capture-macos.sh
• cargo make check-swift
• cargo make fmt-swift-check
• cargo test -p rsnap-host-ffi --lib
• cargo test -p rsnap-overlay --lib live_frame_stream_macos::tests::nonblocking_after_seq_query_does_not_prime_when_same_monitor_already_has_latest_frame
• git diff --check