feat(soak): click-to-watch live video via CDP screencast

Runner creates a Screencaster before sessions are acquired, then
wires its start/stop into DashboardServer.onStartStream / onStopStream
after sessions exist (the handlers close over a sessionsByKey map).
Clicking a player tile in the dashboard starts a CDP screencast on
that session's page, forwards JPEG frames as WS "frame" messages.
Closing the modal (or disconnecting the WS) stops all screencasts.

Verified end-to-end: programmatically connected WS, sent start_stream,
received 5 frames (13.7KB each), sent stop_stream, screencast_stopped
log line fired, run completed cleanly.

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

tests/soak/runner.ts

@@ -16,8 +16,9 @@ import { createLogger } from './core/logger';
 import { SessionPool } from './core/session-pool';
 import { RoomCoordinator } from './core/room-coordinator';
 import { DashboardServer } from './dashboard/server';
+import { Screencaster } from './core/screencaster';
 import { getScenario, listScenarios } from './scenarios';
-import type { DashboardReporter, ScenarioContext } from './core/types';
+import type { DashboardReporter, ScenarioContext, Session } from './core/types';
 
 function noopDashboard(): DashboardReporter {
   return {
@@ -107,37 +108,9 @@ async function main(): Promise<void> {
     logger,
   });
   const coordinator = new RoomCoordinator();
+  const screencaster = new Screencaster(logger);
 
-  // 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') {
+  if (watch === 'tiled') {
     logger.warn('tiled_not_yet_implemented');
     console.warn('Watch mode "tiled" not yet implemented (Task 24). Falling back to none.');
   }
@@ -151,11 +124,64 @@ async function main(): Promise<void> {
   process.on('SIGINT', () => onSignal('SIGINT'));
   process.on('SIGTERM', () => onSignal('SIGTERM'));
 
+  let dashboardServer: DashboardServer | null = null;
+  let dashboard: DashboardReporter = noopDashboard();
   let exitCode = 0;
   try {
     const sessions = await pool.acquire(accounts);
     logger.info('sessions_acquired', { count: sessions.length });
 
+    // Build a session lookup for click-to-watch
+    const sessionsByKey = new Map<string, Session>();
+    for (const s of sessions) sessionsByKey.set(s.key, s);
+
+    // Dashboard with screencaster handlers now that sessions exist
+    if (watch === 'dashboard') {
+      const port = Number(config.dashboardPort ?? 7777);
+      dashboardServer = new DashboardServer(port, logger, {
+        onStartStream: (key) => {
+          const session = sessionsByKey.get(key);
+          if (!session) {
+            logger.warn('stream_start_unknown_session', { sessionKey: key });
+            return;
+          }
+          screencaster
+            .start(key, session.page, (jpegBase64) => {
+              dashboardServer!.broadcast({ type: 'frame', sessionKey: key, jpegBase64 });
+            })
+            .catch((err) =>
+              logger.error('screencast_start_failed', {
+                sessionKey: key,
+                error: err instanceof Error ? err.message : String(err),
+              }),
+            );
+        },
+        onStopStream: (key) => {
+          screencaster.stop(key).catch(() => {
+            // best-effort — errors already logged inside Screencaster
+          });
+        },
+        onDisconnect: () => {
+          screencaster.stopAll().catch(() => {});
+        },
+      });
+      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.
+      }
+    }
+
     const ctx: ScenarioContext = {
       config,
       sessions,
@@ -191,6 +217,7 @@ async function main(): Promise<void> {
     });
     exitCode = 1;
   } finally {
+    await screencaster.stopAll();
     await pool.release();
     if (dashboardServer) {
       await dashboardServer.stop();