fix(dsh-plugin-browserskill): address review findings (leaks, trust fence, queue race)

Blocking:
- B1: delete screenshot scratch PNGs — observation frames reuse one fixed
  per-session path and unlink after every read (finally); browser_screenshot
  unlinks once the bytes are in the attachment store (kept only when the file
  itself is the model-facing artifact).
- B2: replicate dsh's browser-trust fence on /bsk-observation/*: loopback
  Host only, Origin must match Host, sec-fetch-site: cross-site refused, POST
  requires application/json; README documents the loopback trust premise and
  the 0.0.0.0 warning.

Major:
- M1: KeyedExecutor tail chains previous+task (allSettled) so a task aborted
  while queued cannot release the next one into the running session.
- M2: client tracks the live frame per session — replacing/removing/reset
  revokes the old blob URL at once; loads settling after replacement never
  resurrect it.

Minor: drop replaced thumbRefs (+on remove); beginAction fires inside the
queue (no label overwrite while queued, no idle flash); half-initialized
session cleanup stops through the queue with one retry; SSE (re)open refetches
/state (+ snapshot flag renamed subscribed); install probe uses --version (no
daemon spawn); emulate mobile documents the width+height requirement (the
daemon refuses it alone — verified).

Nit: single BskRunOptions declaration; tails map entry dropped on drain; SSE
cleanup on res close; popOut catches requestWindow rejections; resize handle
gains keyboard control (arrows, 16px steps, aria value attrs).

Tests: +14 (fence rules, scratch lifecycle, queue race, blob revocation,
instrumentation timing, SSE resync). 113/113 green.

packages/dsh-plugin-browserskill/tests/client/observation-store.test.ts

@@ -111,3 +111,87 @@ describe("ObservationClientStore", () => {
     expect(store.getSnapshot().sessions).toHaveLength(0);
   });
 });
+
+describe("thumbnail blob URL lifecycle", () => {
+  function withRevokeSpy() {
+    const revoke = vi.fn();
+    (URL as unknown as { revokeObjectURL: unknown }).revokeObjectURL = revoke;
+    return revoke;
+  }
+
+  it("replacing a frame revokes the old blob URL and drops its entry", async () => {
+    const revoke = withRevokeSpy();
+    const { store, esInstances } = harness([{ ...OBS_IDLE, thumbnailAttachmentId: "a1" }]);
+    store.start();
+    await vi.waitFor(() => expect(store.getSnapshot().sessions).toHaveLength(1));
+    store.ensureThumbnail("a1");
+    await vi.waitFor(() =>
+      expect(store.getSnapshot().thumbnails["a1"]).toEqual({ status: "ready", url: "blob:url-a1" }),
+    );
+    emit(esInstances[0], {
+      type: "upsert",
+      session: { ...OBS_IDLE, thumbnailAttachmentId: "a2" },
+    });
+    expect(store.getSnapshot().thumbnails["a1"]).toBeUndefined();
+    expect(revoke).toHaveBeenCalledWith("blob:url-a1");
+    store.stop();
+  });
+
+  it("remove and reset revoke the session's tracked frame", async () => {
+    const revoke = withRevokeSpy();
+    const { store, esInstances } = harness([{ ...OBS_IDLE, thumbnailAttachmentId: "a1" }]);
+    store.start();
+    await vi.waitFor(() => expect(store.getSnapshot().sessions).toHaveLength(1));
+    store.ensureThumbnail("a1");
+    await vi.waitFor(() => expect(store.getSnapshot().thumbnails["a1"].status).toBe("ready"));
+    emit(esInstances[0], {
+      type: "remove",
+      session: { sessionId: "s1", action: "idle", since: 0 },
+    });
+    expect(revoke).toHaveBeenCalledWith("blob:url-a1");
+    expect(store.getSnapshot().thumbnails["a1"]).toBeUndefined();
+    store.stop();
+  });
+
+  it("a load that finishes after replacement never resurrects the old frame", async () => {
+    const revoke = withRevokeSpy();
+    const { store, esInstances, loadImage } = harness([
+      { ...OBS_IDLE, thumbnailAttachmentId: "a1" },
+    ]);
+    let resolveA1!: (url: string) => void;
+    loadImage.mockImplementation((id: string) =>
+      id === "a1"
+        ? new Promise<string>((resolve) => {
+            resolveA1 = resolve;
+          })
+        : Promise.resolve(`blob:url-${id}`),
+    );
+    store.start();
+    await vi.waitFor(() => expect(store.getSnapshot().sessions).toHaveLength(1));
+    store.ensureThumbnail("a1");
+    // the frame is replaced while a1 is still loading
+    emit(esInstances[0], {
+      type: "upsert",
+      session: { ...OBS_IDLE, thumbnailAttachmentId: "a2" },
+    });
+    resolveA1("blob:url-a1-late");
+    await new Promise((resolve) => setImmediate(resolve));
+    expect(store.getSnapshot().thumbnails["a1"]).toBeUndefined();
+    expect(revoke).toHaveBeenCalledWith("blob:url-a1-late");
+    store.stop();
+  });
+});
+
+describe("SSE (re)open resync", () => {
+  it("refetches the full state when the stream (re)opens", async () => {
+    const { store, fetches, esInstances } = harness([OBS_IDLE]);
+    store.start();
+    await vi.waitFor(() => expect(store.getSnapshot().sessions).toHaveLength(1));
+    const before = fetches.filter((f) => f.url === "/bsk-observation/state").length;
+    esInstances[0].onopen?.();
+    await vi.waitFor(() =>
+      expect(fetches.filter((f) => f.url === "/bsk-observation/state").length).toBe(before + 1),
+    );
+    store.stop();
+  });
+});