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.

docs/ARCHITECTURE.md

@@ -572,7 +572,10 @@ renders the next turn without a self-refetch. The `notify` package owns the Flat
 (fed wire-agnostic input structs by the domain services) and the gateway forwards every payload
 verbatim. Auto-match needs no match poll — `Enqueue` returns the game the player enters
 synchronously, and an opponent later taking the open seat arrives as the in-app **opponent-joined**
-event; for the lobby **notification badge** (incoming friend requests + open
+event. Unlike a move, that event has no follow-up delta to trigger the move-count gap recovery, so
+the waiting game screen recovers a missed join itself: it **polls `game.state` while the stream is
+down and refetches once on stream reconnect**, so a join missed during an outage or while the app
+was backgrounded still resolves the open game in place. For the lobby **notification badge** (incoming friend requests + open
 invitations) the client re-polls on the `notify` event and on lobby open / focus, covering a push
 missed while the app was hidden. **Out-of-app platform push** is a fallback
 the **gateway** routes from the same firehose: for an event whose recipient has **no