test(visual): solid white+black window marker to remove subpixel AA drift

[afonsojramos]

Summary

Continuous-improvement step on the visual ROI work. The remaining drift after #28 was 5/14 platforms showing ~1-2% pixel jitter, driven by anti-aliasing at the cyan→yellow boundary line inside the window and the magenta/green checker against macOS's translucent menu bar.

This PR replaces the window content with solid blocks of color:

• Old: 200x100 split horizontally — cyan (top) / yellow (bottom). A subpixel AA seam where the colors meet jittered between runs.
• New: 200x100 white background with a centered solid 80x40 black rectangle. No AA inside the rectangle (pure white-to-white). One sharp AA boundary at the inner square edge, but that's between two solid colors with no transparency, so it should render deterministically.

Tray icon stays as the magenta/green checker — it's the backdrop (macOS menu bar translucency) that causes its drift, which is unfixable without redesigning the icon altogether.

Detection logic moved from "global cyan+yellow pixel count" to "white+black pixel count inside the reported window.getBounds() rect", which also stops OS chrome white/black text from contributing false-positives. Thresholds: ≥5000 white + ≥500 black inside the rect (out of an expected ~16800 + ~3200 fully visible).

Test plan

• All 14 visual jobs pass with the new detection
• After merge: auto-refresh PR diff shrinks from 5 → 0-1 platforms (or close to it)
• Saved screenshots show a white "frame" where the window is (the inner black square blends with the black mask background — that's fine, the frame is the visual cue)