refactor(kernels): extract reusable clipboard and matchprop kernels

Publishes app-free domain logic for shared use: match_special_props and PolylineMatchStyle move to entities::match_props; xdict trio usable headlessly; new match_layer, copy_to_clipboard, paste_entities and match_properties kernels over CadDocument. Handlers stay thin shells keeping undo, dirty flags, selection and command-line echo.

Adds cross-document copy, paste and property-match tests plus a headless proof driving copy, paste and match with no app instance.

src/entities/mod.rs

@@ -15,6 +15,7 @@ pub mod leader;
 pub mod light;
 pub mod line;
 pub mod lwpolyline;
+pub mod match_props;
 pub mod mesh;
 pub mod mline;
 pub mod mtext;