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/src/client/ObservationOverlay.tsx

@@ -434,10 +434,15 @@ export function ObservationOverlay({ store }: { store: ObservationClientStore })
   const popOut = async (): Promise<void> => {
     const pip = pipApi();
     if (pip === undefined) return;
-    const win = await pip.requestWindow({ width: size.w, height: size.h });
-    cloneStylesInto(win);
-    win.addEventListener("pagehide", () => setPipWindow(null));
-    setPipWindow(win);
+    try {
+      const win = await pip.requestWindow({ width: size.w, height: size.h });
+      cloneStylesInto(win);
+      win.addEventListener("pagehide", () => setPipWindow(null));
+      setPipWindow(win);
+    } catch {
+      // requestWindow rejects without a user gesture (or when the window was
+      // closed mid-request): stay on the in-page card, no state change.
+    }
   };
 
   // Hidden while no owned session exists (and no PiP is up).
@@ -496,7 +501,28 @@ export function ObservationOverlay({ store }: { store: ObservationClientStore })
         data-testid="obs-resize"
         aria-label="Resize overlay"
         role="separator"
+        aria-valuenow={size.w}
+        aria-valuetext={`${Math.round(size.w)} by ${Math.round(size.h)} pixels`}
+        aria-valuemin={MIN_SIZE.w}
+        aria-valuemax={Math.round(viewport().w * 0.8)}
+        tabIndex={0}
         onPointerDown={beginDrag("resize")}
+        onKeyDown={(event) => {
+          const step = 16;
+          const delta =
+            event.key === "ArrowRight"
+              ? { w: step, h: 0 }
+              : event.key === "ArrowLeft"
+                ? { w: -step, h: 0 }
+                : event.key === "ArrowDown"
+                  ? { w: 0, h: step }
+                  : event.key === "ArrowUp"
+                    ? { w: 0, h: -step }
+                    : undefined;
+          if (delta === undefined) return;
+          event.preventDefault();
+          setSize(clampSize({ w: size.w + delta.w, h: size.h + delta.h }, viewport()));
+        }}
       />
     </div>
   );