Run the regression suites out of sight and wait for the colour popup

[leafOfTree]

Why

• Test runs flashed popups in the screen corners and could take focus. The suites place their windows off-screen, but WinForms keeps dropdowns and tooltips on a screen, so they land at the top-left of the leftmost monitor (popups and shadows from Architecture, PopupRendering, SettingsEditors, SettingsTheme) or near (32,32) on the primary one (TabInteraction tips and menus, TaskbarPreview).
• SettingsEditors occasionally failed with a NullReferenceException in visiblePopup(): one DoEvents after clicking the colour swatch was not always enough for the popup to show.

What

• tests/Run-Tests.ps1 creates a hidden desktop per run and starts every suite on it (child processes such as PrintWindowHelper inherit it). -VisibleDesktop keeps the old behaviour for watching a run.
• visiblePopup in tests/SettingsEditors.fsx pumps messages for up to 2 s and fails with "Colour popup did not open" if none appears.
• AGENTS.md and docs/testing.md describe the hidden desktop.

Verification

• All 13 suites pass through the new runner; a WinEvent monitor on the user's desktop saw no window from any suite (before the change it logged popups from 8 suites).
• Run-Tests.ps1 -Suites SettingsEditors -Repeat 5 passes 5/5.
• -VisibleDesktop still runs (WindowIcon).
• Not run locally: -Coverage; CI covers it.