feat(adoption): one owner for config/layout identity and the adoption sequence

`src/config-adoption.ts` owns config/layout with revision and fingerprint;
the host keeps `_serverCfg`/`_cfgRev`/`_layout`/`_layoutRev` as delegates.
All seven authoritative adoptions go through `adoptAuthoritativeGated`
(backdrop readiness → continuity → adopt → profile tail); the post-write
paths (space/delete ×2, optimize_undo, import/apply) gain the gate and take
revisions from the re-read bodies. `rollbackOptimistic` moves to the owner;
plan-optimize, space copy and the vacuum writers stop assigning identity.

Issue: #500
User-Visible: no

src/serialized-write-queue.ts

@@ -35,25 +35,5 @@ export function optimisticAttempt<T>(
   };
 }
 
-/**
- * Roll back only the failed candidate represented by this attempt. A conflict
- * reload or a newer edit owns a different revision/content and must win.
- */
-export function rollbackOptimistic<T>(
-  host: {
-    _serverCfg: T | null;
-    _cfgRev: number;
-    _cfgContentFingerprint: string;
-    requestUpdate: () => unknown;
-  },
-  attempt: OptimisticAttempt<T>,
-  fingerprint: (value: T) => string,
-): boolean {
-  const current = host._serverCfg;
-  if (!current || host._cfgRev !== attempt.revision
-      || fingerprint(current) !== attempt.attemptedFingerprint) return false;
-  host._serverCfg = attempt.previous;
-  host._cfgContentFingerprint = attempt.previousFingerprint;
-  host.requestUpdate();
-  return true;
-}
+// The rollback of an attempt lives with the identity owner:
+// `ConfigAdoption.rollbackOptimistic` (#500).