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/tcpd/tcpconnection.c

@@ -31,6 +31,7 @@ int TcpConnection_Init(tcp_connection_t* conn, int sockFd, const struct sockaddr
 
     conn->closing = false;
     conn->disconnectedNotified = false;
+    atomic_init(&conn->pinCount, 0);
     conn->dataBuf = NULL;
     conn->dataBufLen = 0;
     conn->dataBufCap = 0;
@@ -262,6 +263,20 @@ bool TcpConnection_IsDisconnectNotified(tcp_connection_t* conn) {
     return notified;
 }
 
+void TcpConnection_Pin(tcp_connection_t* conn) {
+    if (!conn) {
+        return;
+    }
+    atomic_fetch_add(&conn->pinCount, 1);
+}
+
+void TcpConnection_Unpin(tcp_connection_t* conn) {
+    if (!conn) {
+        return;
+    }
+    atomic_fetch_sub(&conn->pinCount, 1);
+}
+
 static int extract_v4(const tcp_connection_t* conn, struct in_addr* v4out) {
     if (conn->addrFamily == AF_INET6) {
         const struct sockaddr_in6* a6 = (const struct sockaddr_in6*)&conn->peerAddr;