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-intellij/src/main/java/de/neitzel/fxdesigner/intellij/IntellijHost.java

@@ -22,6 +22,7 @@ public final class IntellijHost implements AutoCloseable {
       Project project, HostServices.Runtimes runtimes, HostServices.Diagnostics diagnostics) {
     var docs = scope.own(new IntellijDocuments(project));
     var builds = new ProcessBuilds();
+    var problems = scope.own(new IntellijDiagnostics(project, diagnostics));
     services =
         new HostServices(
             docs,
@@ -62,6 +63,33 @@ public final class IntellijHost implements AutoCloseable {
                             }));
               }
 
+              public CompletionStage<Void> openSource(HostServices.Location location) {
+                var result = new CompletableFuture<Void>();
+                ApplicationManager.getApplication()
+                    .invokeLater(
+                        () ->
+                            WriteIntentReadAction.run(
+                                () -> {
+                                  try {
+                                    if (project.isDisposed())
+                                      throw new IllegalStateException("Project disposed");
+                                    var file =
+                                        LocalFileSystem.getInstance()
+                                            .refreshAndFindFileByNioFile(
+                                                Path.of(location.document()));
+                                    if (file == null)
+                                      throw new IllegalArgumentException(
+                                          "Navigation file unavailable");
+                                    new OpenFileDescriptor(project, file, location.offset())
+                                        .navigate(true);
+                                    result.complete(null);
+                                  } catch (Throwable e) {
+                                    result.completeExceptionally(e);
+                                  }
+                                }));
+                return result;
+              }
+
               public CompletionStage<Void> open(HostServices.Location location) {
                 var result = new CompletableFuture<Void>();
                 ApplicationManager.getApplication()
@@ -107,7 +135,7 @@ public final class IntellijHost implements AutoCloseable {
                 return result;
               }
             },
-            diagnostics,
+            problems,
             scope);
     Disposer.register(project, this::close);
   }