docs(handlers): recommend starting tagName with the app id

Like the files sidebar tabs. The library cannot check it, so it is a
recommendation rather than a rule.

Assisted-by: ClaudeCode:claude-opus-5-5
Signed-off-by: John Molakvoæ <14975046+skjnldsv@users.noreply.github.com>

Author: skjnldsv

README.md

@@ -336,7 +336,9 @@ The full handler shape (see the `IHandler` interface):
 Gotchas:
 
 - `tagName` must be lowercase, contain a hyphen, and have no leading, trailing
-  or consecutive hyphens (e.g. `my-app-viewer`). An invalid one throws.
+  or consecutive hyphens (e.g. `my-app-viewer`). An invalid one throws. Start
+  it with your app id: custom elements share one registry for the whole page,
+  and a name another app already defined is not yours to render.
 - `id` must be unique **across every app on the page**, not just your own:
   it is not namespaced for you. A collision does not throw: the second
   registration is silently dropped with a console warning, so pick something

lib/handlers.ts

@@ -35,6 +35,9 @@ export interface IHandler {
 
 	/**
 	 * The custom element tag name to use for this handler.
+	 *
+	 * Custom elements share one registry for the whole page, so start it
+	 * with your app id, e.g. `myapp-note-view` for the app `myapp`.
 	 */
 	tagName: string