fix(web): serialize live topology mutations + surface failures loudly

Live MazeNET edits fired their mutations fire-and-forget: each canvas
action enqueued immediately and never awaited the result. Two failures
followed from that:

- expected_version is bumped at ENQUEUE (not at apply), so two ops fired
  back-to-back raced — the second carried a stale version and 409'd.
  Edits only worked when hand-paced (an SSE refetch landed between them).
- A failed mutation degrades the topology, but the only signal was a 4s
  toast, so the user saw DEGRADED with no cause.

useTopologyEditor now routes every live op through a serialized submit
queue: one enqueue in flight at a time (submission order preserved), an
optimistic expected_version cursor advanced per enqueue so back-to-back
ops (e.g. reparent's detach+attach) don't need a refetch between them,
and each mutation awaited to a terminal state. A 'failed' row throws
MutationFailedError, which the page pins as a persistent UPDATE FAILED
banner instead of a vanishing toast.

Slice 1 of the live-edit rework; stage+UPDATE-button batching and louder
backend materialisation reporting to follow.

decnet_web/src/components/MazeNET/useTopologyData.ts

@@ -5,6 +5,7 @@ import type { Net, MazeNode, Edge } from './types';
 import { DEFAULT_SERVICES, ARCHETYPES as DEFAULT_ARCHETYPES } from './data';
 import type { Archetype, ServiceDef } from './data';
 import type { MazeApi } from './useMazeApi';
+import { MutationFailedError } from './useTopologyEditor';
 import { useTopologyStream, type TopologyStreamEvent } from './useTopologyStream';
 
 export interface TopoMeta {
@@ -42,6 +43,10 @@ export interface UseTopologyDataResult {
   // Errors + transient banners
   loadErr: string | null;
   actionErr: string | null;
+  /** Persistent (no auto-clear) error from a failed live mutation —
+   *  the topology likely went degraded. Dismissed via clearCommitErr. */
+  commitErr: string | null;
+  clearCommitErr: () => void;
   flashErr: (err: unknown, fallback: string) => void;
 
   // Deploy
@@ -77,9 +82,18 @@ export function useTopologyData(
 
   const [loadErr, setLoadErr] = useState<string | null>(null);
   const [actionErr, setActionErr] = useState<string | null>(null);
+  const [commitErr, setCommitErr] = useState<string | null>(null);
   const [deploying, setDeploying] = useState(false);
 
+  const clearCommitErr = useCallback(() => setCommitErr(null), []);
+
   const flashErr = useCallback((err: unknown, fallback: string) => {
+    // A failed live mutation is loud + persistent: the queue halted and
+    // the topology probably degraded — don't let it vanish in 4s.
+    if (err instanceof MutationFailedError) {
+      setCommitErr(err.message);
+      return;
+    }
     const msg = (err as ApiError)?.response?.data?.detail ?? (err as ApiError)?.message ?? fallback;
     setActionErr(msg);
     setTimeout(() => setActionErr(null), 4000);
@@ -189,7 +203,7 @@ export function useTopologyData(
     edges, setEdges,
     topoMeta,
     services, archetypes,
-    loadErr, actionErr, flashErr,
+    loadErr, actionErr, commitErr, clearCommitErr, flashErr,
     deploying, onDeploy,
     streamLive, lastEventAt, streamEnabled,
     refetch,