fix: Rebuild projections once per context after the registry changed

Every ephemeral OfflineSyncDatabase shares one context but initializes a
recorder of its own, and each rebuilt the projections of every space
when the context's first initialize had changed the registry: on every
session and every merge, until the process stopped (upstream #150). A
new device, or one whose app changed its synced tables, spent about 3 s
of every inbound merge on it with a library of 8,000 cards, a server
after such a migration as much on every request.

The context now runs the rebuild once (Once): wrappers initializing
while it runs await the same run, later ones skip it, and a failed run
is tried again by the next.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/serverpod_offline_sync/lib/src/utils/once.dart

@@ -0,0 +1,21 @@
+import 'package:meta/meta.dart';
+
+/// Runs an action once for everyone who asks: a caller while it runs awaits
+/// the same run, and one after it is done gets it done; only a run that
+/// failed is run again by the next caller.
+@internal
+class Once {
+  Future<void>? _run;
+
+  /// Runs [action] unless it ran or runs already, and completes with that
+  /// run.
+  Future<void> run(Future<void> Function() action) async {
+    final run = _run ??= action();
+    try {
+      await run;
+    } catch (_) {
+      if (identical(_run, run)) _run = null;
+      rethrow;
+    }
+  }
+}