Document a mixed-DPI test matrix

[ajhcs]

Create a reusable display-testing guide covering 100%, 125%, 150%, and 200% scaling across primary and secondary monitor positions.

Acceptance criteria

• Add docs/testing-displays.md with the test matrix.
• Cover monitors positioned left, right, and above the primary display, including mixed scaling.
• Link the physical-bounds regression and manual calibration flow.
• Explain setup, expected physical bounds, commands, and sanitized evidence requirements.
• Include no private desktop captures.

Helpful starting points

• tests/DpiPreviewRegression.ps1
• docs/configuration.md
• CONTRIBUTING.md