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/tcpd/tcpconnection.h

@@ -3,6 +3,7 @@
 
 #include <arpa/inet.h>
 #include <pthread.h>
+#include <stdatomic.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
@@ -37,6 +38,11 @@ struct tcp_connection_t {
     bool closing;
     bool disconnectedNotified;
 
+    // Non-zero while another thread holds a raw pointer to this connection taken from a
+    // lock-protected snapshot and used after releasing the lock. The reaper must not free a
+    // pinned connection. See TcpConnection_Pin/Unpin.
+    atomic_int pinCount;
+
     unsigned char* dataBuf;
     size_t dataBufLen;
     size_t dataBufCap;
@@ -75,4 +81,9 @@ void TcpConnection_RequestClose(tcp_connection_t* conn);
 void TcpConnection_MarkDisconnectNotified(tcp_connection_t* conn);
 bool TcpConnection_IsDisconnectNotified(tcp_connection_t* conn);
 
+// Pin/unpin a connection so a background reaper won't free it while a caller still holds a raw
+// pointer to it (e.g. across a blocking operation after releasing the collection lock).
+void TcpConnection_Pin(tcp_connection_t* conn);
+void TcpConnection_Unpin(tcp_connection_t* conn);
+
 #endif