keyboard: the one-Esc law — a surfaced layer claims at the document level (keyboard 4 of 4)

Owner ruling (keyboard round 2026-10-06): Esc (or any key) does ONE
action, on the topmost active layer. The plan-open-while-dictating
double-act was the symptom: the plan popup listened at WINDOW level
with no claim, and same-node window listeners all fire — the global
chain (registered first) cancelled the dictation while the plan's own
listener closed the plan. One Esc, two actions.

The law (now the house pattern, documented at the chain):
- field-level handlers claim at their element (focus in a field: the
  field's action) — unchanged, already correct
- every SURFACED layer claims at the DOCUMENT level with preventDefault
  + stopPropagation — document fires before the window chain, so the
  chain stays silent under any surface
- the window chain is the bottom-up fallback: dictation (a live capture,
  NOT a surface — it survives the Esc that closes a plan or overlay),
  then abort (escAbortTarget layers its own targets)

Converted to the claim: plan popup, session overlay, chain table,
settings dialog, setup dialog, the delete/abort dialogs (window-level
stopPropagation was never enough — same-node listeners all fire), and
the stats popover. Popovers (popover-dismiss), menus, model picker, and
the sidebar context menu already claimed correctly.

1427 tests, tsc, lint, build green.

components/session-overlay.tsx

@@ -74,13 +74,19 @@ export function MermaidOverlay({ chart, onClose }: { chart: string; onClose: ()
 		};
 	}, [chart, attempt]);
 
-	// Esc closes.
+	// Esc closes — claimed at the document level (THE ONE-ESC LAW, owner
+	// keyboard round 2026-10-06): one Esc, one action; the window chain
+	// (dictation → abort) never fires under a surfaced layer.
 	useEffect(() => {
 		const onKey = (e: KeyboardEvent) => {
-			if (e.key === "Escape") onClose();
+			if (e.key === "Escape") {
+				e.preventDefault();
+				e.stopPropagation();
+				onClose();
+			}
 		};
-		window.addEventListener("keydown", onKey);
-		return () => window.removeEventListener("keydown", onKey);
+		document.addEventListener("keydown", onKey);
+		return () => document.removeEventListener("keydown", onKey);
 	}, [onClose]);
 
 	// ── pan/zoom viewport (same wiring as the plan popup) ──