fix(auth): keep oauth-persist browser-safe (move Node file backend out)

The refactor routed the browser's RemoteOAuthStorage through
`oauth-persist.ts`, which value-imported the Node-only `store-io.js`
(`atomically` -> `stubborn-fs` -> `node:process.getuid`, plus
`node:fs/promises`). That dragged Node built-ins into the browser bundle
and the web app crashed to a blank page at runtime — a regression from
v2/main, where browser OAuth storage used a Zustand JSON adapter with no
Node imports. Unit/integration tests run in node/happy-dom and
`smoke:web` only checks the served HTML, so none of them execute the app
in a real browser and the break slipped through.

Fix, following the existing `store-id.ts` precedent:
- Split the pure JSON (de)serializers into Node-free
  `core/storage/store-serialize.ts`; `store-io.ts` re-exports them so
  existing importers are unaffected.
- Move the Node-only `createFileOAuthPersistBackend` into
  `core/auth/node/oauth-persist-file.ts`; update `storage-node.ts` and
  the storage adapters test to import from there.
- `oauth-persist.ts` now imports only the Node-free serializers, so the
  browser bundle no longer pulls in `node:fs`/`atomically`.

Tests:
- oauth-persist-browser-safe.test.ts: source-level guard that
  `oauth-persist.ts` never imports store-io/node:/atomically.
- Direct unit tests for the isomorphic remote + session backends
  (restoring oauth-persist.ts coverage after the file backend moved out).
- store.test.ts: OAuthMemoryStore unit coverage incl. replace() fallbacks.
- storage.test.ts: clearAllOAuthClientState null-snapshot (no file) path.

Verified end-to-end in a real browser (Playwright): DCR OAuth flow
completes, tokens persist via the remote store, and after a full page
reload the server reconnects with no re-authentication.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

core/auth/node/oauth-persist-file.ts

@@ -0,0 +1,42 @@
+/**
+ * Node-only file backend for OAuth persistence. Kept out of the isomorphic
+ * `core/auth/oauth-persist.ts` because it imports `store-io.js` (which pulls
+ * `node:fs`/`atomically`); the browser must never load that. Node consumers
+ * (e.g. `NodeOAuthStorage`) import the file backend from here, while the
+ * shared blob (de)serialization and browser/remote backends stay isomorphic.
+ */
+
+import {
+  readStoreFile,
+  writeStoreFile,
+  deleteStoreFile,
+} from "../../storage/store-io.js";
+import {
+  parseOAuthPersistBlob,
+  serializeOAuthPersistBlob,
+  type OAuthPersistBackend,
+} from "../oauth-persist.js";
+
+export interface FileOAuthPersistBackendOptions {
+  filePath: string;
+}
+
+export function createFileOAuthPersistBackend(
+  options: FileOAuthPersistBackendOptions,
+): OAuthPersistBackend {
+  return {
+    async read() {
+      const raw = await readStoreFile(options.filePath);
+      return parseOAuthPersistBlob(raw);
+    },
+    async write(snapshot) {
+      await writeStoreFile(
+        options.filePath,
+        serializeOAuthPersistBlob(snapshot),
+      );
+    },
+    async remove() {
+      await deleteStoreFile(options.filePath);
+    },
+  };
+}

core/auth/node/storage-node.ts

@@ -1,6 +1,6 @@
 import { OAuthStorageBase } from "../oauth-storage.js";
 import { OAuthMemoryStore } from "../store.js";
-import { createFileOAuthPersistBackend } from "../oauth-persist.js";
+import { createFileOAuthPersistBackend } from "./oauth-persist-file.js";
 import {
   getDefaultStorageDir,
   getStoreFilePath,