fix(display): keep blackout across reloads and follow the display's current assignment

A kiosk that reloaded during a blackout restored its last frame from local
state without the blackout, so the hidden slide flashed on the wall until the
server answered.

A display resolved its channel once, at registration. Moving it to another
group or audience, or deleting the channel it was on, never reached a running
screen: it kept showing the old channel, or fell into offline continuation
against a 404 forever. The display now fetches its state by viewer, so the
server resolves the current assignment on every fetch, and the heartbeat reply
names the assigned channel so a reassignment lands within one heartbeat. A
display removed from the roster registers again once, on backoff; only
Signage's own not-found means removal, so a foreign 404 can never cause a
registration loop. The channel route stays for the preview and for display
code a kiosk cached earlier. A replaced realtime subscription can no longer
schedule a stray resubscription when it reports closing.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/shared/display-protocol.ts

@@ -206,9 +206,28 @@ export function parseDisplayHeartbeat(raw: unknown): ProtocolResult<DisplayHeart
 export interface DisplayHeartbeatResponse {
   readonly acknowledged: true;
   readonly recorded: boolean;
+  /** The channel an administrator has this viewer assigned to now. A display whose own idea of its
+   * channel differs refetches its state, so a reassignment reaches a running screen within one
+   * heartbeat. Optional: a display ignores a response without it. */
+  readonly channelId?: string;
 }
 
-// --- Channel state: GET channels/:channelId/display-state -----------------------------------------
+/** The assigned channel from a heartbeat response, if it carried a usable one. */
+export function assignedChannelOf(raw: unknown): string | undefined {
+  if (typeof raw !== "object" || raw === null) {
+    return undefined;
+  }
+  const channelId = (raw as { readonly channelId?: unknown }).channelId;
+  return typeof channelId === "string" && channelId.length > 0 && channelId.length <= 64
+    ? channelId
+    : undefined;
+}
+
+// --- Channel state: GET viewers/:viewerId/display-state (and channels/:channelId/display-state) -----
+//
+// A registered display asks by its own viewer id, so the server resolves which channel it is assigned
+// to at that moment. The channel route remains for the management preview and for display code a
+// kiosk cached before the viewer route existed.
 
 /** A fresh, signed delivery URL. A bearer capability with an expiry: never persisted by a display. */
 export interface DisplayMedia {