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/core/session-pool.ts

@@ -142,12 +142,20 @@ export interface SessionPoolOptions {
   /** Optional override — if absent, SessionPool launches its own. */
   browser?: Browser;
   contextOptions?: Parameters<Browser['newContext']>[0];
+  /**
+   * If > 0, the first `headedHostCount` sessions use a separate headed
+   * Chromium browser with visible windows (positioned in a 2×2 grid
+   * via window.moveTo). The remaining sessions stay headless. Used for
+   * --watch=tiled mode.
+   */
+  headedHostCount?: number;
 }
 
 export class SessionPool {
   private accounts: Account[] = [];
   private ownedBrowser: Browser | null = null;
   private browser: Browser | null;
+  private headedBrowser: Browser | null = null;
   private activeSessions: Session[] = [];
 
   constructor(private opts: SessionPoolOptions) {
@@ -221,6 +229,10 @@ export class SessionPool {
   /**
    * Launch the browser if not provided, create N contexts, log each in via
    * localStorage injection, return the live sessions.
+   *
+   * If `headedHostCount > 0`, launches a second headed Chromium and
+   * places the first `headedHostCount` sessions in it, positioning
+   * their windows in a 2×2 grid. The rest stay headless.
    */
   async acquire(count: number): Promise<Session[]> {
     await this.ensureAccounts(count);
@@ -229,13 +241,52 @@ export class SessionPool {
       this.browser = this.ownedBrowser;
     }
 
+    const headedCount = this.opts.headedHostCount ?? 0;
+    if (headedCount > 0 && !this.headedBrowser) {
+      this.headedBrowser = await chromium.launch({
+        headless: false,
+        slowMo: 50,
+      });
+    }
+
     const sessions: Session[] = [];
     for (let i = 0; i < count; i++) {
       const account = this.accounts[i];
-      const context = await this.browser.newContext(this.opts.contextOptions);
+      const useHeaded = i < headedCount;
+      const targetBrowser = useHeaded ? this.headedBrowser! : this.browser!;
+      // Headed host windows get a larger viewport — 960×900 fits the full
+      // game table (deck + opponent row + own 2×3 grid + status area) on
+      // a typical 1920×1080 display. Two windows side-by-side still fit
+      // horizontally; if the user runs more than 2 rooms in tiled mode
+      // the extra windows will overlap and need to be arranged manually.
+      const context = await targetBrowser.newContext({
+        ...this.opts.contextOptions,
+        ...(useHeaded ? { viewport: { width: 960, height: 900 } } : {}),
+      });
       await this.injectAuth(context, account);
       const page = await context.newPage();
       await page.goto(this.opts.targetUrl);
+
+      // Best-effort tile placement. window.moveTo is often a no-op on
+      // modern Chromium (especially under Wayland), so we don't rely on
+      // it — the viewport sized above is what the user actually sees.
+      if (useHeaded) {
+        const col = i % 2;
+        const row = Math.floor(i / 2);
+        const x = col * 960;
+        const y = row * 920;
+        await page
+          .evaluate(
+            ([x, y]) => {
+              window.moveTo(x, y);
+            },
+            [x, y] as [number, number],
+          )
+          .catch(() => {
+            // ignore — window.moveTo may be blocked
+          });
+      }
+
       const bot = new GolfBot(page);
       sessions.push({ account, context, page, bot, key: account.key });
     }
@@ -282,7 +333,7 @@ export class SessionPool {
     }
   }
 
-  /** Close all active contexts. Safe to call multiple times. */
+  /** Close all active contexts + browsers. Safe to call multiple times. */
   async release(): Promise<void> {
     for (const session of this.activeSessions) {
       try {
@@ -301,5 +352,13 @@ export class SessionPool {
       this.ownedBrowser = null;
       this.browser = null;
     }
+    if (this.headedBrowser) {
+      try {
+        await this.headedBrowser.close();
+      } catch {
+        // ignore
+      }
+      this.headedBrowser = null;
+    }
   }
 }