Phase 2 (refactor-commit-12..18) created Controls/ with 6 UserControls that
split MainWindow.xaml, but no index.md existed — a gap against the schema.md
rule (one index.md per code directory). Documents each control, the window↔
control contract, and the Phase-2 landmines (window-resources converter rule,
control xmlns, name-looked-up host stubs, dropped-Border).
