test(server): cover mid-run ServerSet listener failure

ServerSet's startup/bind-failure paths were already covered, but nothing
exercised a listener failing after every listener had bound and was
actively serving. Inject the failure through the crate's own established
token-source test seam (the same one core::tests already uses) via a
narrow #[cfg(test)] Server::from_socket_with_core constructor, so one
listener answers a first open like its healthy sibling before its second
draw fails deterministically. Confirms ServerSet reports the failure,
shuts the sibling down, joins both tasks and releases both sockets, with
no external shutdown ever sent.

crates/irtt-server/src/runtime.rs

@@ -109,6 +109,28 @@ impl Server {
         socket: UdpSocket,
         config: ServerConfig,
     ) -> Result<Self, ServerRuntimeError> {
+        Self::with_core(socket, ServerCore::new(config))
+    }
+
+    /// Test-only seam: builds a listener around an already-constructed core
+    /// instead of one this method would build from a [`ServerConfig`].
+    ///
+    /// This exists so a unit test can substitute a scripted
+    /// [`TokenSource`](crate::token::TokenSource) and make one listener in a
+    /// [`ServerSet`](crate::ServerSet) fail deterministically after it has
+    /// already bound and answered real traffic — the runtime otherwise offers
+    /// no way to make a core failure happen on command without OS-level
+    /// socket sabotage. Every other code path here — wildcard setup, kernel
+    /// timestamp configuration — is unchanged from [`Server::from_socket`].
+    #[cfg(test)]
+    pub(crate) fn from_socket_with_core(
+        socket: UdpSocket,
+        core: ServerCore,
+    ) -> Result<Self, ServerRuntimeError> {
+        Self::with_core(socket, core)
+    }
+
+    fn with_core(socket: UdpSocket, core: ServerCore) -> Result<Self, ServerRuntimeError> {
         let addr = socket
             .local_addr()
             .map_err(|source| ServerRuntimeError::LocalAddr { source })?;
@@ -131,7 +153,7 @@ impl Server {
 
         Ok(Self {
             socket,
-            core: ServerCore::new(config),
+            core,
             recv_buffer: vec![0; RECEIVE_BUFFER_LEN],
             listener_is_ipv4: marks_with_ipv4_option(addr),
             select_reply_source,