Add PixelGrid (pure-Rust test grid) and PixelWriter trait for unit
testing line algorithms without WA runtime. Add snapshot capture in
DLL (OPENWA_CAPTURE_LINE_SNAPSHOTS=1, triggered at frame 10) that
draws test lines on a BitGrid using WA's native functions and saves
pixel data for comparison.
Also fix headful test crash: remove --minimized flag which became
incompatible with WA's DirectDraw initialization.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
