Add CultMeshQuicNativeRuntime.retain(): a synchronous extra reference

A caller that already holds a live runtime instance (the QUIC realtime
provider, handing a reference to each newly accepted peer) needs one more
reference without crossing an async boundary. `open()` itself has no I/O
to await, but being `async` still forces every caller through a microtask
hop; `retain()` just increments the shared refcount and returns `this`,
synchronously, so the caller stays inside the same turn.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

packages/cultmesh-ts/src/realtime-quic-native.ts

@@ -296,6 +296,19 @@ export class CultMeshQuicNativeRuntime {
     return CultMeshQuicNativeRuntime.shared;
   }
 
+  /**
+   * Takes one more reference on this already-open runtime, synchronously —
+   * unlike `open()`, this never awaits a runtime-open call, so a caller
+   * already holding a live instance (a provider handing a reference to a
+   * newly accepted peer) can attach it in the same synchronous turn instead
+   * of crossing a microtask boundary that would leave the event a caller is
+   * mid-dispatching unowned until the next tick.
+   */
+  retain(): CultMeshQuicNativeRuntime {
+    CultMeshQuicNativeRuntime.refCountValue += 1;
+    return this;
+  }
+
   /**
    * Drops one reference; closes the shared runtime once none remain. Refuses
    * a release when no reference is outstanding. The bridge header (section 4)