feat: F5.4 presence: who else is in the project

Over Yjs awareness each editor tells who it is (F names the person in
the access message), the device it has chosen and the windows it has
open, and nothing else: no pointer, nothing that changes without a
cause, sent only when it changed. Others' states are read with care.
A frame in their colour round a device they chose, a dot on a window
they have open too, a circle with initials each beside the mode
buttons; the bridge tells the dashboard who is there.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/protocol/src/sync.ts

@@ -43,4 +43,6 @@ export interface AccessMessage {
   readonly readOnly: boolean;
   /** The project is frozen (its deadline passed): no one writes, whatever their level. */
   readonly frozen: boolean;
+  /** Who this is (from the dashboard's ticket), told when the connection comes: the editor shows it to the others (plan §7.4). */
+  readonly you?: { readonly user: string; readonly name: string; readonly color: string };
 }