Model page: Registry panel lists registry files, elements inside each

- one collapsible header per .archify-registry file with its element
  count and an Open button that opens the registry view; files start
  collapsed, the Registry panel open
- view types moved to views/viewTypes.ts so the diagram page can open a
  registry without a circular import
- 0.7.1

src/views/diagramView.ts

@@ -14,9 +14,9 @@ import { handleWebviewMessage, WebviewMessage } from '../webview/messages';
 import { makeNonce, messagePage, modelPage, prepareDiagramHtml } from '../webview/page';
 import { viewer3dOption, viewerRoots } from '../webview/viewer3d';
 
-export const PREVIEW_VIEW_TYPE = 'archify.preview';
-/** Default editor for .archify files: the rendered diagram. */
-export const DIAGRAM_EDITOR_VIEW_TYPE = 'archify.diagram';
+import { DIAGRAM_EDITOR_VIEW_TYPE, PREVIEW_VIEW_TYPE, REGISTRY_VIEW_TYPE } from './viewTypes';
+
+export { DIAGRAM_EDITOR_VIEW_TYPE, PREVIEW_VIEW_TYPE };
 
 interface PreviewState {
   uri: string;
@@ -59,7 +59,11 @@ class DiagramPreview implements vscode.Disposable {
           return;
         }
         if (message.type === 'openDiagram') {
-          void this.openLinked(message.path);
+          void this.openLinked(message.path, DIAGRAM_EDITOR_VIEW_TYPE);
+          return;
+        }
+        if (message.type === 'openRegistry') {
+          void this.openLinked(message.file, REGISTRY_VIEW_TYPE);
           return;
         }
         void handleWebviewMessage(message, this.uri, this.panel);
@@ -96,7 +100,7 @@ class DiagramPreview implements vscode.Disposable {
   }
 
   /** Opens a diagram of the model, given its path relative to the model root. */
-  private async openLinked(relativePath: string): Promise<void> {
+  private async openLinked(relativePath: string, viewType: string): Promise<void> {
     if (!this.root) return;
     const target = vscode.Uri.joinPath(this.root, ...relativePath.split('/'));
     try {
@@ -105,7 +109,7 @@ class DiagramPreview implements vscode.Disposable {
       vscode.window.showWarningMessage(`Archify: ${relativePath} does not exist in this model.`);
       return;
     }
-    await vscode.commands.executeCommand('vscode.openWith', target, DIAGRAM_EDITOR_VIEW_TYPE, vscode.ViewColumn.Active);
+    await vscode.commands.executeCommand('vscode.openWith', target, viewType, vscode.ViewColumn.Active);
   }
 
   private showModelPage(model: ModelPanel): void {