Registry view with element dialog; Elements and Connectors panels under diagrams

- .archify-registry opens in a new read-only custom editor (archify.registry,
  default): the file's elements as cards; click one for a dialog with its
  description, the defining line (opens the YAML) and the diagrams that use
  it (open on click); Show Source and Open Model work from the view
- the same dialog opens from the registry cards on the model page
- Archify's node index is titled Elements; a Connectors panel built from the
  SVG edges lists from → to with the label, highlights on hover and keeps the
  highlight on click; side by side in the bottom rail
- integration test for the registry view; docs and changelog

src/webview/messages.ts

@@ -10,6 +10,7 @@ export type WebviewMessage =
   | { type: 'openRelative'; href: string }
   | { type: 'openSource'; path: string; line?: number; endLine?: number; href?: string }
   | { type: 'openDiagram'; path: string }
+  | { type: 'openRegistry'; file: string; line?: number }
   | { type: 'error'; message: string };
 
 /** Column for opening files next to a webview without covering it. */