fix(menu): open submenus on hover and keep them inside the viewport

[kirillDevPro]

Hovering an enabled submenu row now opens its child and hovering another row switches or closes the branch. Moving into the child keeps it open. Nested menus use measured row bounds, flip left at the right edge, and cap their dimensions to the viewport.

Validation:

• MoonUI guardrails: 453 component tests, 7 gallery tests, formatting, component audit, API and donor-mirror checks passed.
• Hover and placement regression tests cover Dark/Light themes, eager and virtualized menus, disabled rows, leaf clicks, and three levels. Both regressions were mutation-proven.
• MoonTerminal workspace tests passed with the local dependency patch; the Windows executable was built and shipped.
• Independent review found no correctness defects.
• Gallery screenshot comparison remains red against older checked-in baselines (Dark 7.36%, Light 99.99%); current images were inspected and baselines were left unchanged. Native MoonTerminal interaction has not been verified here.