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

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 - **Models:** a folder with a `main.archify` at its top is a model, a set of diagrams that belong together. Opening `main.archify` shows the model page: name and description, the overview diagram if the file has one, the diagrams grouped by folder (click to open), and the registry with where each element is used.
 - **Registry:** `*.archify-registry` files define shared elements once; diagrams draw them with `ref: <id>` plus position, and may override `label`, `sublabel` and `tag`. Hover, go-to-definition and completion on `ref`; unknown names, forbidden overrides, duplicate ids and broken registries in Problems. Registry edits re-render open diagrams.
+- **Registry view:** `.archify-registry` files open as their elements (cards). Clicking an element opens a dialog with its description, the registry line that defines it and the diagrams that use it; the same dialog opens from the registry cards on the model page. *Show Source* opens the YAML beside the view.
+- **Elements and Connectors:** the node index under a diagram is titled *Elements*; a new *Connectors* panel lists every connection (from → to, label), highlights it on hover and keeps it highlighted on click.
 - **Model page panels:** Diagrams and Registry are collapsible panels; inside Diagrams every folder has its own collapsible header above its diagram cards. Open or closed is remembered.
 - **Cards:** a model diagram without `cards` of its own gets Archify cards from the registry descriptions of the elements it draws; the model page shows the registry as the same cards, each with the diagrams it is used in.
 - **Links:** `diagram:` on a registry element or a component points at the diagram that explains it; double-click the node (2D view) to open it.