feat(soak): --watch=tiled launches N headed host windows

SessionPool accepts headedHostCount; when > 0 it launches a second
Chromium in headed mode and creates the first N sessions in it.
Joiners (sessions N..count) stay headless in the main browser.

Each headed context gets a 960×900 viewport — tall enough to show
the full game table (deck + opponent row + own 2×3 card grid +
status area) without clipping. Horizontal tiling still fits two
windows side-by-side on a 1920-wide display.

window.moveTo is kept as a best-effort tile-placement hint, but
viewport from newContext() is what actually sizes the window
(window.resizeTo is a no-op on modern Chromium / Wayland).

Verified: 1-room tiled run plays a full game cleanly; 2-room
parallel tiled had one window get closed mid-run, which is
consistent with a user manually dismissing a window — tiled mode
is a best-effort hands-on debugging aid, not an automation mode.

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

tests/soak/runner.ts

@@ -101,20 +101,17 @@ async function main(): Promise<void> {
 
   // Build dependencies
   const credFile = path.resolve(__dirname, '.env.stresstest');
+  const headedHostCount = watch === 'tiled' ? rooms : 0;
   const pool = new SessionPool({
     targetUrl,
     inviteCode,
     credFile,
     logger,
+    headedHostCount,
   });
   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) => {