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 {
@@ -107,19 +108,61 @@ async function main(): Promise<void> {
     logger,
   });
   const coordinator = new RoomCoordinator();
+  const screencaster = new Screencaster(logger);
+
+  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) => {
+    logger.warn('signal_received', { signal: sig });
+    abortController.abort();
+  };
+  process.on('SIGINT', () => onSignal('SIGINT'));
+  process.on('SIGTERM', () => onSignal('SIGTERM'));
 
-  // Optional dashboard server
   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) => {
-        logger.info('stream_start_requested', { sessionKey: key });
-        // Wired in Task 23
+          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) => {
-        logger.info('stream_stop_requested', { sessionKey: key });
+          screencaster.stop(key).catch(() => {
+            // best-effort — errors already logged inside Screencaster
+          });
+        },
+        onDisconnect: () => {
+          screencaster.stopAll().catch(() => {});
         },
       });
       await dashboardServer.start();
@@ -137,25 +180,8 @@ async function main(): Promise<void> {
       } 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) => {
-    logger.warn('signal_received', { signal: sig });
-    abortController.abort();
-  };
-  process.on('SIGINT', () => onSignal('SIGINT'));
-  process.on('SIGTERM', () => onSignal('SIGTERM'));
-
-  let exitCode = 0;
-  try {
-    const sessions = await pool.acquire(accounts);
-    logger.info('sessions_acquired', { count: sessions.length });
-
     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();