fix(ui): poll/refetch fallback for a missed opponent_joined in the open-game wait

PR #51 moved the auto-match "wait for an opponent" from the lobby matchmaking
screen into the open game but did not carry over that screen's poll fallback.
The notify hub is best-effort and never replays, the live-stream resubscribe
sends no cursor, and the game screen refreshed only from push events — so an
opponent_joined dropped while the stream was down (e.g. a mobile suspend) left
the starter stuck on "searching for opponent" until they re-entered the game.
Unlike opponent_moved/game_over, opponent_joined has no follow-up event to
trigger the existing move-count gap refetch.

Recover it in Game.svelte: (A) refetch once on stream reconnect (covers the
common suspend/resume case and rescues a missed move/game_over too), and
(B) poll game.state every 2.5s while still waiting with the stream down
(mirrors the old matchmaking startPoll). Add a mock-mode __stream e2e seam and
two specs isolating each path, fix the now-stale streamAlive comment, and
document the fallback in ARCHITECTURE §10.

ui/src/lib/app.svelte.ts

@@ -36,7 +36,9 @@ export interface Toast {
 
 export const app = $state<{
   ready: boolean;
-  /** Whether the live-event stream is connected; drives the matchmaking poll fallback. */
+  /** Whether the live-event stream is connected. The event hub is best-effort and never replays a
+   *  missed event, so an open game waiting for an opponent uses this to recover: it polls the game
+   *  state while the stream is down and refetches once on reconnect (see game/Game.svelte). */
   streamAlive: boolean;
   session: Session | null;
   profile: Profile | null;
@@ -500,3 +502,14 @@ if (typeof window !== 'undefined') {
 }
 telegramOnEvent('activated', goForeground);
 telegramOnEvent('deactivated', goBackground);
+
+// Mock-mode e2e seam (tree-shaken from a production build): drive the live-stream lifecycle so the
+// e2e can exercise the open-game fallbacks for a missed opponent_joined. `drop` tears the stream
+// down without scheduling a reconnect — so only the in-game poll can then recover — and `restore`
+// reopens it (the reconnect catch-up path). Never wired outside mock mode.
+if (import.meta.env.MODE === 'mock' && typeof window !== 'undefined') {
+  (window as unknown as { __stream?: { drop(): void; restore(): void } }).__stream = {
+    drop: closeStream,
+    restore: openStream,
+  };
+}