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/index.ts

@@ -8,9 +8,11 @@
 
 import type { Scenario } from '../core/types';
 import populate from './populate';
+import stress from './stress';
 
 const registry: Record<string, Scenario> = {
   populate,
+  stress,
 };
 
 export function getScenario(name: string): Scenario | undefined {