Tooling: forma-xaml validate --layout and a ui_layout_problems MCP tool #13

Lay a view out with sample data across scales and locales (including the pseudo-locale) and print FindLayoutProblems findings with file:line and help; a game MCP tool returns the current screen's findings; optional debug inspector highlight.

Part of the plan in Trace Arena's docs/layout-problems-plan.md (catching clipped and overflowing text in every language). Additive: no breaking public API.