feat(soak): wire --watch=dashboard in runner

Starts DashboardServer on port 7777 (or --dashboard-port), uses its
reporter as ctx.dashboard, auto-opens the URL via xdg-open/open/start.
Cleans up on exit. WS client connections logged as info events so
you can see when the browser attaches.

Verified: 2-account populate run with --watch=dashboard serves the
static page on :7777, accepts WS connections, cleanly shuts down
when the run completes.

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

tests/soak/runner.ts

@@ -10,10 +10,12 @@
  */
 
 import * as path from 'path';
+import { spawn } from 'child_process';
 import { parseArgs, mergeConfig, CliArgs } from './config';
 import { createLogger } from './core/logger';
 import { SessionPool } from './core/session-pool';
 import { RoomCoordinator } from './core/room-coordinator';
+import { DashboardServer } from './dashboard/server';
 import { getScenario, listScenarios } from './scenarios';
 import type { DashboardReporter, ScenarioContext } from './core/types';
 
@@ -96,11 +98,6 @@ async function main(): Promise<void> {
     return;
   }
 
-  if (watch !== 'none') {
-    logger.warn('watch_mode_not_yet_implemented', { watch });
-    console.warn(`Watch mode "${watch}" not yet implemented — falling back to "none".`);
-  }
-
   // Build dependencies
   const credFile = path.resolve(__dirname, '.env.stresstest');
   const pool = new SessionPool({
@@ -110,7 +107,41 @@ async function main(): Promise<void> {
     logger,
   });
   const coordinator = new RoomCoordinator();
-  const dashboard = noopDashboard();
+
+  // Optional dashboard server
+  let dashboardServer: DashboardServer | null = null;
+  let dashboard: DashboardReporter = noopDashboard();
+  if (watch === 'dashboard') {
+    const port = Number(config.dashboardPort ?? 7777);
+    dashboardServer = new DashboardServer(port, logger, {
+      onStartStream: (key) => {
+        logger.info('stream_start_requested', { sessionKey: key });
+        // Wired in Task 23
+      },
+      onStopStream: (key) => {
+        logger.info('stream_stop_requested', { sessionKey: key });
+      },
+    });
+    await dashboardServer.start();
+    dashboard = dashboardServer.reporter();
+    const url = `http://localhost:${port}`;
+    console.log(`Dashboard: ${url}`);
+    try {
+      const opener =
+        process.platform === 'darwin'
+          ? 'open'
+          : process.platform === 'win32'
+            ? 'start'
+            : 'xdg-open';
+      spawn(opener, [url], { stdio: 'ignore', detached: true }).unref();
+    } catch {
+      // If auto-open fails, the URL is already printed.
+    }
+  } else if (watch === 'tiled') {
+    logger.warn('tiled_not_yet_implemented');
+    console.warn('Watch mode "tiled" not yet implemented (Task 24). Falling back to none.');
+  }
+
   const abortController = new AbortController();
 
   const onSignal = (sig: string) => {
@@ -161,6 +192,9 @@ async function main(): Promise<void> {
     exitCode = 1;
   } finally {
     await pool.release();
+    if (dashboardServer) {
+      await dashboardServer.stop();
+    }
   }
 
   if (abortController.signal.aborted && exitCode === 0) exitCode = 2;