fix(maestro): mobile drawer access, focus preservation, stale rAF focus (#140)

Three more fixes from acceptance review:

- structured.ts's header omitted the .sessions-btn attach.ts's terminal
  header carries. The sidebar is off-canvas on mobile, so a structured
  session had no way to reach it — add the same button and
  requestToggleDrawer wiring.

- ConnectionPool.reconcileViews always remounted a repaired connection
  with focus:false. If the mistyped pane was the one actually focused,
  disposing it dropped DOM focus while `focusedId` still named it. Pass
  `focus: this.focusedId === id` instead — `focusedId` only ever names a
  real focus (multiview.ts skips setFocus while previewing), so a
  preview or a merely-visible split neighbour still comes back
  unfocused.

- onMount's deferred requestAnimationFrame focus had no way to tell it
  had been superseded by an unmount (or a remount elsewhere) before it
  fired, so a parked pane could steal focus a frame later. A token
  bumped on every mount/unmount lets the callback recognize it's stale
  and no-op.

Regression tests: 7 new (structured.test.ts, connections.test.ts). 495
tests total.

web/src/structured.ts

@@ -12,6 +12,7 @@ import {
   sendStructuredPrompt,
   structuredWsUrl,
 } from "./api.js";
+import { requestToggleDrawer } from "./drawer.js";
 import type { CatalogState, CatalogStore } from "./events.js";
 import { paneTitleAttr, paneTitleHtml, type AttachInfo } from "./paneTitle.js";
 import type { PaneConnection } from "./pane.js";
@@ -57,6 +58,7 @@ export function createStructuredConnection(
   el.className = "attach structured";
   el.innerHTML = `
     <header class="attach-header">
+      <button type="button" class="btn btn-ghost btn-icon sessions-btn" aria-label="Show sessions" title="Show sessions">☰</button>
       <span class="attach-title" title="${paneTitleAttr(sessionId, info)}">${paneTitleHtml(sessionId, info)}</span>
       <span class="st-pills"></span>
       <span class="attach-state" data-state="connecting">connecting…</span>
@@ -87,6 +89,12 @@ export function createStructuredConnection(
   const cancelBtn = el.querySelector(".structured-cancel") as HTMLButtonElement;
 
   disconnectBtn.addEventListener("click", () => opts.onRequestClose?.(sessionId));
+  // Mobile-only (CSS hides it on desktop, where the sidebar is always in
+  // view): the session list is an off-canvas drawer and this opens it —
+  // same control attach.ts's terminal header carries.
+  (el.querySelector(".sessions-btn") as HTMLElement).addEventListener("click", () =>
+    requestToggleDrawer(),
+  );
 
   const setConnState = (text: string, kind: "connecting" | "open" | "closed" | "error") => {
     stateEl.textContent = text;
@@ -346,17 +354,26 @@ export function createStructuredConnection(
   }
   connect();
 
+  // A pane can be parked again before its deferred focus frame fires (a
+  // preview stepped past it, a split closed it) — the frame must not focus a
+  // hidden textarea out from under whatever the operator moved to since.
+  // `mountToken` is bumped on every mount/unmount so a stale frame can tell
+  // it is no longer the most recent one.
+  let mountToken = 0;
+
   return {
     sessionId,
     el,
     onMount(mountOpts?: { focus?: boolean }) {
       const focusOnMount = mountOpts?.focus !== false;
+      const token = ++mountToken;
       requestAnimationFrame(() => {
+        if (token !== mountToken) return; // parked/remounted since this was scheduled
         if (focusOnMount && !input.disabled) input.focus();
       });
     },
     onUnmount() {
-      // Nothing to release: the composer needs no GPU context or size fit.
+      mountToken++;
     },
     focusTerminal() {
       if (!input.disabled) input.focus();