Fix native scroll capture stitching

[yvette-carlisle]

Summary:

• route scroll capture into the native host and keep the selected viewport live while stitching
• add fixed-area minimap preview and stitched-image copy/save/OCR export
• harden pairwise stitching so ambiguous or mismatched overlap does not append a broken seam

Validation:

• cargo test -p rsnap-overlay worker_pairwise_vision
• cargo make fmt-check
• cargo make test-host-reset
• cargo make test-macos-native-host
• git diff --check