Optimize scroll export and gate short scroll selections

[yvette-carlisle]

Summary

• keep scroll export/copy-save on direct RGBA paths and avoid cloning stitched RGBA just to read dimensions
• reduce frozen overlay compositor work by delaying spotlight clones, restoring spotlights by rows, and bounding stroke coverage masks to dirty rects
• disable Scroll Capture entry for selections under 120 px high, including the toolbar button and plain s shortcut

Validation

• cargo make check
• scripts/perf/local.sh (frozen_overlay_export_rgba_1440x900 77.384ms / 900ms)
• ./scripts/build_and_run.sh --verify