Implement network manager simulator lifecycle (#53)

crates/libremetaverse/src/client_core.rs

@@ -223,12 +223,23 @@ impl ClientRuntime {
 /// Construction is side-effect free: it creates no runtime, task, socket, or
 /// HTTP request. Network-facing services are attached explicitly and are owned
 /// until ordered shutdown or drop.
+#[derive(Clone)]
 pub struct GridClient {
     pub(crate) settings: Settings,
     pub(crate) time_provider: TimeProvider,
     runtime: Arc<ClientRuntime>,
 }
 
+#[cfg(test)]
+pub(crate) struct ClientRetentionProbe(std::sync::Weak<ClientRuntime>);
+
+#[cfg(test)]
+impl ClientRetentionProbe {
+    pub(crate) fn is_released(&self) -> bool {
+        self.0.upgrade().is_none()
+    }
+}
+
 impl GridClient {
     #[must_use]
     pub fn builder() -> GridClientBuilder {
@@ -301,6 +312,11 @@ impl GridClient {
     pub(crate) fn shutdown(&self) -> Result<(), ClientCoreError> {
         self.runtime.shutdown()
     }
+
+    #[cfg(test)]
+    pub(crate) fn retention_probe(&self) -> ClientRetentionProbe {
+        ClientRetentionProbe(Arc::downgrade(&self.runtime))
+    }
 }
 
 impl fmt::Debug for GridClient {
@@ -325,7 +341,9 @@ impl fmt::Debug for GridClient {
 
 impl Drop for GridClient {
     fn drop(&mut self) {
-        let _ = self.runtime.shutdown();
+        if Arc::strong_count(&self.runtime) == 1 {
+            let _ = self.runtime.shutdown();
+        }
     }
 }
 
@@ -432,6 +450,11 @@ impl Settings {
         }
     }
 
+    /// Mutable access to the mapped agent settings for native composition.
+    pub fn agent_settings_mut(&mut self) -> &mut AgentSettings {
+        &mut self.agent
+    }
+
     #[must_use]
     pub fn connection_mut(&mut self) -> &mut ConnectionSettings {
         &mut self.connection