feat(soak): stress scenario with chaos injection

Rapid short games with a parallel chaos loop that has a 5% per-turn
chance of firing one of:
  - rapid_clicks: 5 quick clicks at the player's own cards
  - tab_blur:    window blur/focus event pair
  - brief_offline: 300ms network outage via context.setOffline

Chaos counts roll up into ScenarioResult.customMetrics.chaos_fired.

Important detail: chaos loop has a 3-second initial delay so room
creation, joiners, and game start can complete without interference.
Chaos during lobby setup (especially brief_offline) was causing
#create-room-btn to go unstable.

Verified: stress smoke with --games-per-room=3, 4 accounts + 1 CPU,
first game completed with 37 turns and chaos events fired across all
three event types.

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

tests/soak/scenarios/shared/chaos.ts

@@ -0,0 +1,65 @@
+/**
+ * Chaos injector — occasionally fires unexpected UI events while a
+ * game is playing, to hunt race conditions and recovery bugs.
+ *
+ * Called from the stress scenario's background chaos loop. Each call
+ * has `probability` of firing; when it fires it picks one random
+ * event type and runs it against one session.
+ */
+
+import type { Session, Logger } from '../../core/types';
+
+export type ChaosEvent = 'rapid_clicks' | 'tab_blur' | 'brief_offline';
+
+const ALL_EVENTS: ChaosEvent[] = ['rapid_clicks', 'tab_blur', 'brief_offline'];
+
+function pickEvent(): ChaosEvent {
+  return ALL_EVENTS[Math.floor(Math.random() * ALL_EVENTS.length)];
+}
+
+export async function maybeInjectChaos(
+  session: Session,
+  probability: number,
+  logger: Logger,
+  roomId: string,
+): Promise<ChaosEvent | null> {
+  if (Math.random() >= probability) return null;
+
+  const event = pickEvent();
+  logger.info('chaos_injected', { room: roomId, session: session.key, event });
+  try {
+    switch (event) {
+      case 'rapid_clicks': {
+        // Fire 5 rapid clicks at the player's own cards
+        for (let i = 0; i < 5; i++) {
+          await session.page
+            .locator(`#player-cards .card:nth-child(${(i % 6) + 1})`)
+            .click({ timeout: 300 })
+            .catch(() => {});
+        }
+        break;
+      }
+      case 'tab_blur': {
+        // Briefly dispatch blur then focus — simulates user tabbing away
+        await session.page.evaluate(() => {
+          window.dispatchEvent(new Event('blur'));
+          setTimeout(() => window.dispatchEvent(new Event('focus')), 200);
+        });
+        break;
+      }
+      case 'brief_offline': {
+        // 300ms network outage — should trigger client reconnect logic
+        await session.context.setOffline(true);
+        await new Promise((r) => setTimeout(r, 300));
+        await session.context.setOffline(false);
+        break;
+      }
+    }
+  } catch (err) {
+    logger.warn('chaos_error', {
+      event,
+      error: err instanceof Error ? err.message : String(err),
+    });
+  }
+  return event;
+}