fix(app): preserve stroke fringe in dirty redraws

[kivutar]

Summary

• expand the framework-managed dirty clip by one logical pixel
• preserve centered one-pixel borders and their antialiasing fringe during hover and focus redraws
• add regression coverage for the padded dirty clip

Why

Framework-managed incremental redraw used the exact dirty widget bounds as its clip. Widget painters commonly center strokes on those bounds, so the clip removed the outer part of the stroke and its antialiasing fringe. This made button and text-field borders disappear during incremental hover or focus updates, especially at fractional scale.

The fix retains incremental rendering and adds only a one-logical-pixel margin around the existing dirty union; it does not force a full redraw.

Validation

• reproduced with the Vulkan renderer at 1.25x scale in a nested widget tree
• verified button-hover and text-field-focus redraws retain their borders
• go test ./...
• go vet ./...