Clean up scroll capture architecture

[yvette-carlisle]

Summary

• Split native scroll-capture observation from session control and move Rust stitching internals into focused fingerprint/types/worker modules.
• Align docs and smoke surfaces with the fail-closed native scroll-capture behavior.
• Add toolbar liquid-glass cadence telemetry and smoke checks without changing the restored visual refresh path.

Validation

• cargo make check
• cargo test -p rsnap-overlay scroll_capture --lib
• cargo make check-swift
• git diff --check
• native scroll smoke with proof-stripe continuity and codex-like fail-closed coverage