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  had been superseded by an unmount (or a remount elsewhere) before it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.test.ts

@@ -1,5 +1,5 @@
 // @vitest-environment happy-dom
-import { beforeEach, describe, expect, it, vi } from "vitest";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 
 vi.mock("./api.js", async () => {
   const actual = await vi.importActual<typeof import("./api.js")>("./api.js");
@@ -12,6 +12,7 @@ vi.mock("./api.js", async () => {
 });
 
 import { cancelStructuredTurn, resolveStructuredApproval, sendStructuredPrompt } from "./api.js";
+import { TOGGLE_DRAWER_EVENT } from "./drawer.js";
 import { CatalogStore } from "./events.js";
 import { createStructuredConnection, type StructuredSocketFactory } from "./structured.js";
 import type { ReducedState, Session, StructuredEvent, TranscriptRow } from "./types.js";
@@ -349,6 +350,22 @@ describe("structured connection: pane lifecycle", () => {
     conn.dispose();
   });
 
+  it("carries the same off-canvas sessions button as the terminal attach header", () => {
+    const conn = mount({ sockets: [new FakeSocket()] });
+    const btn = conn.el.querySelector(".sessions-btn") as HTMLButtonElement;
+    expect(btn).not.toBeNull();
+    expect(btn.getAttribute("aria-label")).toBe("Show sessions");
+    expect(btn.type).toBe("button");
+
+    let toggled = 0;
+    const onToggle = () => toggled++;
+    document.addEventListener(TOGGLE_DRAWER_EVENT, onToggle);
+    btn.click();
+    document.removeEventListener(TOGGLE_DRAWER_EVENT, onToggle);
+    expect(toggled).toBe(1);
+    conn.dispose();
+  });
+
   it("focusTerminal focuses the composer only when it is enabled", () => {
     const ws = new FakeSocket();
     const conn = mount({ sockets: [ws] });
@@ -400,3 +417,72 @@ describe("structured connection: pane lifecycle", () => {
     conn.dispose();
   });
 });
+
+describe("structured connection: deferred mount focus", () => {
+  let frames: FrameRequestCallback[];
+
+  beforeEach(() => {
+    vi.clearAllMocks();
+    frames = [];
+    vi.stubGlobal("requestAnimationFrame", (cb: FrameRequestCallback) => {
+      frames.push(cb);
+      return frames.length;
+    });
+  });
+
+  afterEach(() => vi.unstubAllGlobals());
+
+  function runFrames(): void {
+    const pending = frames;
+    frames = [];
+    for (const cb of pending) cb(0);
+  }
+
+  it("does not focus the composer if the pane was parked before its deferred frame ran", () => {
+    const ws = new FakeSocket();
+    const conn = mount({ sockets: [ws] });
+    ws.open();
+    ws.emit({ type: "transcript_snapshot", snapshot: { cursor: 0, rows: [] } });
+    ws.emit({ type: "reduced_state", state: idleState });
+
+    conn.onMount({ focus: true });
+    conn.onUnmount(); // parked again before the frame below runs
+    runFrames();
+
+    const input = conn.el.querySelector(".structured-input") as HTMLTextAreaElement;
+    expect(document.activeElement).not.toBe(input);
+    conn.dispose();
+  });
+
+  it("does not focus the composer if it was re-mounted into a different pane before the stale frame ran", () => {
+    const ws = new FakeSocket();
+    const conn = mount({ sockets: [ws] });
+    ws.open();
+    ws.emit({ type: "transcript_snapshot", snapshot: { cursor: 0, rows: [] } });
+    ws.emit({ type: "reduced_state", state: idleState });
+
+    conn.onMount({ focus: true }); // stale frame #1
+    conn.onUnmount();
+    conn.onMount({ focus: false }); // the pane that actually won focus, without it
+    runFrames(); // both frames fire; only the latest should have any say
+
+    const input = conn.el.querySelector(".structured-input") as HTMLTextAreaElement;
+    expect(document.activeElement).not.toBe(input);
+    conn.dispose();
+  });
+
+  it("still focuses normally when nothing raced the frame", () => {
+    const ws = new FakeSocket();
+    const conn = mount({ sockets: [ws] });
+    ws.open();
+    ws.emit({ type: "transcript_snapshot", snapshot: { cursor: 0, rows: [] } });
+    ws.emit({ type: "reduced_state", state: idleState });
+
+    conn.onMount({ focus: true });
+    runFrames();
+
+    const input = conn.el.querySelector(".structured-input") as HTMLTextAreaElement;
+    expect(document.activeElement).toBe(input);
+    conn.dispose();
+  });
+});