fix(windows): rewrite the preference store in place when a reader vetoes the rename

dart:io opens files without FILE_SHARE_DELETE, so MoveFileExW with
MOVEFILE_REPLACE_EXISTING - and any other replacement strategy - fails
while such a reader holds the document open. Fall back to the upstream
in-place rewrite so a hostile reader costs at most crash-atomicity for
that one write instead of silently dropping it, and sweep the staging
copy the rename did not consume.

packages/shared_preferences_windows/lib/shared_preferences_windows.dart

@@ -427,7 +427,29 @@ Future<bool> _writePreferences(
       staged.parent.createSync(recursive: true);
     }
     staged.writeAsStringSync(stringMap, flush: true);
-    staged.renameSync(localDataFile.path);
+    try {
+      staged.renameSync(localDataFile.path);
+    } on FileSystemException {
+      // PLEZY DELTA: a reader that denies delete sharing vetoes every rename
+      // over the document. `MoveFileExW` with MOVEFILE_REPLACE_EXISTING — and
+      // even a POSIX-semantics rename — fails while an open handle on the
+      // destination lacks FILE_SHARE_DELETE, and dart:io's own `File.open`
+      // produces exactly such a handle (`_wopen` shares read/write only), as
+      // do some antivirus and indexing services. The atomic path is not slow
+      // there; it is unavailable.
+      //
+      // Fall back to the upstream in-place rewrite: the sharing mode that
+      // blocks the rename still admits opening the document for write. This
+      // reintroduces the truncate window only while such a reader holds the
+      // store — a brief loss of crash-atomicity beats turning every
+      // preference write into a silent no-op on exactly the machines whose
+      // scanners are most likely to have damaged the store in the first
+      // place.
+      localDataFile.writeAsStringSync(stringMap, flush: true);
+      // The staging copy holds the whole document, credentials included, and
+      // was not consumed by a rename, so it must not outlive the write.
+      _removeStagingFile(localDataFile);
+    }
   } catch (e) {
     debugPrint('Error saving preferences to disk: $e');
     return false;