Heal & Scratch tool enhancements: exclusivity, overlay alignment, tool menus, suspend/restore

[linkmodo]

Summary

A set of fixes and workflow enhancements for the Heal and Scratch tools, aimed at making retouching self-contained on the canvas instead of round-tripping to the sidebar for every correction.

Fixes

• Heal/Scratch buttons are now mutually exclusive. Activating one while the other was on left both highlighted: the tool-sync pass never refreshed the retouch sidebar, so the stale button kept its checked state. The colour sidebar's WB picker had the same latent bug — also synced now.
• Placed-heal overlays now sit exactly where the user clicked/drew. The raw→screen inverse uv-grid lookup snapped to the centres of a ~100-bucket decimated grid (up to half a bucket off — several to tens of pixels depending on preview size, magnified by zoom), while the heal itself used the exact forward lookup. A full-resolution refinement pass over the winning bucket's neighbourhood pins outlines to sub-pixel at any zoom, HQ or not (measured worst case: 1.9px on a 3000px-wide zoomed view over a rotated + cropped HQ grid, ~23px before). The dodge/burn mask overlay and the new heal hit-test share the same lookup and benefit equally.
• Manual heals/scratches now light the Finish tab's edited dot — manual_heal_strokes (where both tools commit) was missing from the Retouch section's modified count.
• Tool placements are left-click only, so right-click no longer drops a stray scratch point or heal on its way to the context menu.

Enhancements

• Tab-away suspend/restore: navigating off the tab hosting Retouch suspends an active heal/scratch tool (canvas clicks stop placing heals while the controls are out of sight); returning restores the suspended tool and its overlay. Nothing is restored if no tool was active when leaving, or if another tool was picked up meanwhile.
• Pen-nib cursor while the scratch tool is active (rotated to read like a pointer, hotspot on the tip), so click-points line-drawing mode is visible at a glance.
• Tool-specific context menus replace the general settings menu while a heal tool is live. Heal: Undo Last Heal, Clear All Heals. Scratch adds Confirm Scratch (same as double-click/Enter) and Undo Last Point. Right-clicking a placed heal offers Delete This Heal — a bad patch can be picked off directly instead of unwinding newer heals step by step.
• Backspace steps back one click-point of the in-progress scratch polyline.
• Context-sensitive Ctrl+Z: while a heal tool is active it removes the last placed heal; outside the tools it stays the normal edit undo.
• Clear All Heals asks for confirmation (sidebar button and menus) — unlike single-heal undo it is not step-recoverable.
• Tooltips updated to surface the new interactions (right-click to delete, Backspace for points).

Testing

Verified in-app on Windows (tool switching, tab suspend/restore, overlay alignment at multiple zooms with HQ on/off, context menus, Backspace/Ctrl+Z, delete-heal hit-testing). Headless checks cover the raw→screen round-trip precision (≤2px on a rotated/cropped HQ grid), heal hit-testing, and scratch point undo. Targeted suites pass (test_retouch_logic, test_desktop_session, test_compare_baseline — 70 tests); ruff + compile clean.