Comparer: show pixel-level differences as a pink overlay

[chammoru]

Migrated from the legacy TODO list in Comparer/README.md.

Goal:

• Visualize pixel-level differences between compared sources.
• Show differing pixels or regions with a pink/highlight overlay.

Notes:

• This is intended for comparison/diff visualization, not for displaying user-marked or saved regions.

[chammoru]

Original TODO text: "픽셀단위로 틀린 부분 보여주기 (분홍색 overlay로 표시)"

[chammoru]

Implemented in commit f3d2a2d (PR #38). Comparer now overlays a translucent pink rectangle outline plus a center dot in every grid cell that contains differing pixels between the two source panes. Cell size is fixed in display pixels, so zooming in subdivides the source area per cell — at maximum zoom each dot resolves to a single differing source pixel. The overlay is hidden automatically once zoom is high enough to render per-pixel value labels.

D toggles the overlay; ? opens the new Comparer shortcut panel.