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.

include/nets/nodediscovery.h

@@ -23,6 +23,10 @@ void NodeDiscovery_OnGetPeers(node_discovery_t* disc, tcp_connection_t* fromConn
 // Decode a received PEERS payload and fold a couple of its endpoints into the known-peer table.
 void NodeDiscovery_OnPeersReceived(node_discovery_t* disc, tcp_connection_t* fromConn, const unsigned char* payload, size_t payloadLen);
 
+// Strike a peer (by its listen endpoint) from the known-peer table. Called when a peer becomes
+// logically disconnected (no remaining connection to it).
+void NodeDiscovery_RemovePeer(node_discovery_t* disc, const struct sockaddr_storage* endpoint);
+
 // Dump the known-peer table to stdout (for the CLI `peers` command).
 void NodeDiscovery_PrintPeers(node_discovery_t* disc);