Replace pixel_search GetPixel with GetDIBits buffer scan

[mark-e-deyoung]

Problem

pixel_search iterates pixel-by-pixel calling GetPixel() — a GDI round-trip per pixel. A full-screen 1920×1080 region = 2M GDI calls taking seconds.

Proposed Solution

Use GetDIBits to capture the entire region into a memory buffer (one GDI call), then scan the buffer in-memory. Trade-off: ~8MB memory for full-screen 32-bit capture, released immediately after search. Speedup: 100-1000× for large regions.

References

• Performance audit item 5
• core/src/win32_backend.cpp:439-458