Fix UI verification state isolation

[Daliys]

Summary

• Scope MCP test-window verification state to each window instance.
• Preserve that state across UI Toolkit rebuilds.
• Add regression coverage for rebuild behavior.

Root cause

Shared static fields were reset whenever CreateGUI rebuilt the visual tree, allowing unrelated windows and rebuilds to clobber verification results.

Validation

• PYTHONDONTWRITEBYTECODE=1 scripts/prepush-validate.sh --static-only
• Push-hook validation, including Python bridge tests and the quality gate.

Closes #89.