feat(soak): per-room watchdog + heartbeat wiring + multi-game lobby fix

Watchdog class with 4 Vitest tests (27 total now), wired into
ctx.heartbeat in the runner. One watchdog per room with a 60s
timeout; firing logs an error, marks the room's dashboard tile
as errored, and triggers the abort signal so the scenario unwinds.
Watchdogs are explicitly stopped in the runner's finally block
so pending timers don't keep the node process alive on exit.

Also fixes a multi-game bug discovered during stress scenario
verification: after a game ends sessions stay parked on the
game_over screen, which hides the lobby and makes a subsequent
#create-room-btn click time out. runOneMultiplayerGame now
navigates every session to / before each game — localStorage
auth persists so nothing re-logs in.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/soak/scenarios/shared/multiplayer-game.ts

@@ -49,6 +49,12 @@ export async function runOneMultiplayerGame(
   const maxDuration = opts.maxDurationMs ?? 5 * 60_000;
 
   try {
+    // Reset every session back to the lobby before starting.
+    // After the first game ends each session is parked on the
+    // game_over screen, which hides the lobby's Create Room button.
+    // goto('/') bounces them back; localStorage-cached auth persists.
+    await Promise.all(sessions.map((s) => s.bot.goto('/')));
+
     // Host creates game and announces the code
     const code = await host.bot.createGame(host.account.username);
     ctx.coordinator.announce(opts.roomId, code);