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/stress.ts

@@ -0,0 +1,171 @@
+/**
+ * Stress scenario — rapid short games with chaos injection.
+ *
+ * Partitions sessions into N rooms (default 4), runs gamesPerRoom
+ * short 1-hole games per room in parallel. While each game plays,
+ * a background loop injects chaos events (rapid clicks, tab blur,
+ * brief offline) with 5% per-turn probability to hunt race
+ * conditions and recovery bugs.
+ */
+
+import type {
+  Scenario,
+  ScenarioContext,
+  ScenarioResult,
+  ScenarioError,
+  Session,
+} from '../core/types';
+import { runOneMultiplayerGame } from './shared/multiplayer-game';
+import { maybeInjectChaos } from './shared/chaos';
+
+interface StressConfig {
+  gamesPerRoom: number;
+  holes: number;
+  decks: number;
+  rooms: number;
+  cpusPerRoom: number;
+  thinkTimeMs: [number, number];
+  interGamePauseMs: number;
+  chaosChance: number;
+}
+
+function chunk<T>(arr: T[], size: number): T[][] {
+  const out: T[][] = [];
+  for (let i = 0; i < arr.length; i += size) out.push(arr.slice(i, i + size));
+  return out;
+}
+
+async function sleep(ms: number): Promise<void> {
+  return new Promise((r) => setTimeout(r, ms));
+}
+
+async function runStressRoom(
+  ctx: ScenarioContext,
+  cfg: StressConfig,
+  roomIdx: number,
+  sessions: Session[],
+): Promise<{ completed: number; errors: ScenarioError[]; chaosFired: number }> {
+  const roomId = `room-${roomIdx}`;
+  let completed = 0;
+  let chaosFired = 0;
+  const errors: ScenarioError[] = [];
+
+  for (let gameNum = 0; gameNum < cfg.gamesPerRoom; gameNum++) {
+    if (ctx.signal.aborted) break;
+
+    ctx.dashboard.update(roomId, { game: gameNum + 1, totalGames: cfg.gamesPerRoom });
+
+    // Background chaos loop — runs concurrently with the game turn loop.
+    // Delay the first tick by 3 seconds so room creation + joiners + game
+    // start have time to complete without chaos interference (rapid clicks
+    // or brief_offline during lobby setup can prevent #create-room-btn
+    // from becoming stable).
+    let chaosActive = true;
+    const chaosLoop = (async () => {
+      await sleep(3000);
+      while (chaosActive && !ctx.signal.aborted) {
+        await sleep(500);
+        for (const session of sessions) {
+          const e = await maybeInjectChaos(
+            session,
+            cfg.chaosChance,
+            ctx.logger,
+            roomId,
+          );
+          if (e) chaosFired++;
+        }
+      }
+    })();
+
+    const result = await runOneMultiplayerGame(ctx, sessions, {
+      roomId,
+      holes: cfg.holes,
+      decks: cfg.decks,
+      cpusPerRoom: cfg.cpusPerRoom,
+      thinkTimeMs: cfg.thinkTimeMs,
+    });
+
+    chaosActive = false;
+    await chaosLoop;
+
+    if (result.completed) {
+      completed++;
+      ctx.logger.info('game_complete', {
+        room: roomId,
+        game: gameNum + 1,
+        turns: result.turns,
+      });
+    } else {
+      errors.push({
+        room: roomId,
+        reason: 'game_failed',
+        detail: result.error,
+        timestamp: Date.now(),
+      });
+      ctx.logger.error('game_failed', { room: roomId, error: result.error });
+    }
+
+    await sleep(cfg.interGamePauseMs);
+  }
+
+  return { completed, errors, chaosFired };
+}
+
+const stress: Scenario = {
+  name: 'stress',
+  description: 'Rapid short games for stability & race condition hunting',
+  needs: { accounts: 16, rooms: 4, cpusPerRoom: 2 },
+  defaultConfig: {
+    gamesPerRoom: 50,
+    holes: 1,
+    decks: 1,
+    rooms: 4,
+    cpusPerRoom: 2,
+    thinkTimeMs: [50, 150],
+    interGamePauseMs: 200,
+    chaosChance: 0.05,
+  },
+
+  async run(ctx: ScenarioContext): Promise<ScenarioResult> {
+    const start = Date.now();
+    const cfg = ctx.config as unknown as StressConfig;
+    const perRoom = Math.floor(ctx.sessions.length / cfg.rooms);
+    if (perRoom * cfg.rooms !== ctx.sessions.length) {
+      throw new Error(
+        `stress: ${ctx.sessions.length} sessions does not divide evenly into ${cfg.rooms} rooms`,
+      );
+    }
+    const roomSessions = chunk(ctx.sessions, perRoom);
+
+    const results = await Promise.allSettled(
+      roomSessions.map((s, idx) => runStressRoom(ctx, cfg, idx, s)),
+    );
+
+    let gamesCompleted = 0;
+    let chaosFired = 0;
+    const errors: ScenarioError[] = [];
+    results.forEach((r, idx) => {
+      if (r.status === 'fulfilled') {
+        gamesCompleted += r.value.completed;
+        chaosFired += r.value.chaosFired;
+        errors.push(...r.value.errors);
+      } else {
+        errors.push({
+          room: `room-${idx}`,
+          reason: 'room_threw',
+          detail: r.reason instanceof Error ? r.reason.message : String(r.reason),
+          timestamp: Date.now(),
+        });
+      }
+    });
+
+    return {
+      gamesCompleted,
+      errors,
+      durationMs: Date.now() - start,
+      customMetrics: { chaos_fired: chaosFired },
+    };
+  },
+};
+
+export default stress;