Publish shared FXML diagnostics and integrate IntelliJ Problems

Navigate authoritative FXML locations, withdraw stale results and preserve host Undo/Redo. Include editor ownership in native problem identity so equal diagnostics from separate owners survive independent cleanup. Qualify both hosts automatically.

designer-core/src/main/java/de/neitzel/fxdesigner/host/HostServices.java

@@ -74,6 +74,11 @@ public record HostServices(
   public interface Navigation {
     CompletionStage<Void> open(Location location);
 
+    /** Open authoritative source at an offset, rather than selecting the visual designer. */
+    default CompletionStage<Void> openSource(Location location) {
+      return open(location);
+    }
+
     /** Editable attached sources whose owning project is known to the host. */
     default CompletionStage<List<Location>> componentSources(String type) {
       return java.util.concurrent.CompletableFuture.completedFuture(List.of());
@@ -98,6 +103,16 @@ public record HostServices(
 
   public interface Diagnostics {
     void report(Diagnostic diagnostic);
+
+    /** Replace one editor's current diagnostics, never append obsolete validation results. */
+    default void replace(
+        String owner,
+        de.neitzel.fxdesigner.contracts.DocumentSnapshot source,
+        String context,
+        List<Diagnostic> diagnostics) {}
+
+    /** Withdraw this editor's diagnostics without removing another editor's results. */
+    default void clear(String owner) {}
   }
 
   public record BuildRequest(