fix(windows): honor dirty regions when presenting software canvas frames

[oshtz]

Summary

• retain the Windows reference-render surface between compatible software-canvas presents
• round and clamp dirty bounds outward, then rasterize and convert only the damaged pixels after the first full frame
• invalidate and blit only the dirty paint band while keeping the retained DIB at a stable origin
• add Windows mapping and reference-raster regression coverage

Frame-budget impact

Before this change, every update converted and invalidated the entire surface. After the initial full frame, work scales with the damaged rectangle: on the issue's 718x759 reproduction, a representative 718x64 repaint band processes 45,952 pixels instead of 544,962, a 91.6% reduction in conversion work for that update.

Testing

• zig build test-canvas
• zig build test-desktop-platform: 182 passed, 1 skipped, 6 existing failures; clean main reports 181 passed, 1 skipped, the same 6 failures
• scripts/gate.sh fast origin/main (the broad Windows checkout remains red only in failures reproduced on clean main and local example dependency/symlink setup)

Fixes #236

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