Reap dead outbound connection slots; strike disconnected peers from discovery. Two related connection-lifecycle fixes on top of the SIGPIPE fix.

Reclaim outbound slots on peer disconnect:
- TcpClient_ThreadProc fired on_disconnect but never cleared the outbound
  slot, closed the fd, joined the io thread, or freed the connection, so a
  dead peer permanently held its outboundClients[] slot. After MAX_CONS (32)
  churned connections the node could make no new outbound connections, and
  leaked fds/threads/memory. (The inbound side already self-reclaimed.)
- Add a reaper (Node_ReapDeadOutbound) on the maintenance thread: under
  outboundLock it detaches dead (disconnect-notified) slots, then joins the
  io thread and destroys/frees each connection outside the lock.
- Guard against use-after-free with a pin count on tcp_connection_t
  (TcpConnection_Pin/Unpin). The only cross-thread consumer holding a raw
  connection pointer across a blocking op is the `sync` command (via
  Node_GetBestOutboundPeer); it now pins the peer and unpins when done, and
  the reaper skips pinned connections. Discovery's snapshots run on the
  reaper's own thread, so they need no pin.
- Node_GetBestOutboundPeer/GetClientList/GetPeerEndpoints skip
  disconnect-notified connections so a dead peer is never handed out.
- Node_Destroy stops+joins the maintenance thread before tearing down
  outbound clients, so the reaper can't race shutdown.

Strike disconnected peers from the discovery peer list:
- Add NodeDiscovery_RemovePeer + Node_HandlePeerDisconnect: on disconnect,
  remove the peer from the known-peer table once no live connection (inbound
  or outbound) to its listen endpoint remains (Node_HasLiveConnectionTo;
  disconnect-notified conns don't count, so both directions dropping at once
  is handled). Wired into Node_Server_OnDisconnect and Node_Client_OnDisconnect.

src/nets/nodediscovery.c

@@ -274,6 +274,24 @@ void NodeDiscovery_OnPeersReceived(node_discovery_t* disc, tcp_connection_t* fro
     pthread_mutex_unlock(&disc->lock);
 }
 
+void NodeDiscovery_RemovePeer(node_discovery_t* disc, const struct sockaddr_storage* endpoint) {
+    if (!disc || !endpoint) return;
+    pthread_mutex_lock(&disc->lock);
+    size_t n = DynArr_size(disc->peers);
+    for (size_t i = 0; i < n; ++i) {
+        discovered_peer_t* p = (discovered_peer_t*)DynArr_at(disc->peers, i);
+        if (Discovery_AddrEqual(&p->addr, endpoint)) {
+            char ip[INET6_ADDRSTRLEN] = {0};
+            unsigned short port = 0;
+            Discovery_AddrToIpPort(&p->addr, ip, sizeof(ip), &port);
+            printf("NodeDiscovery: struck disconnected peer %s:%u from peer list\n", ip, port);
+            DynArr_remove(disc->peers, i);
+            break;
+        }
+    }
+    pthread_mutex_unlock(&disc->lock);
+}
+
 // ---- periodic tick -----------------------------------------------------------------------
 
 void NodeDiscovery_Iterate(node_discovery_t* disc) {