Add NodeDiscovery: multi-hop peer crawl with UDP-ping preference

- Implement NodeDiscovery engine: known-peer table (DynArr) with a
  per-tick seed/ping/query/connect state machine driven by the node
  maintenance thread; bounded multi-hop crawl (FANOUT peers per node,
  hop-capped) that connects to reachable peers lowest-ping-first
- Add GET_PEERS/PEERS TCP opcodes for peer-list exchange, handled on
  both inbound and outbound connections
- Measure UDP round-trip time and pass it to the on_pong callback
  (previously the send timestamp was only used for retries)
- Advertise each node's listen port in HELLO/ACK_HELLO and store it
  per-connection, so inbound-only peers and non-default ports are
  discoverable (length-guarded parse; wire-compatible with old peers)
- Wire a udp_node_t + node_discovery_t into net_node_t: init/start in
  Node_Create, tick in the maintenance loop, teardown in Node_Destroy
  (stop UDP before destroying discovery to avoid callback races)
- Add Node_ConnListenEndpoint / Node_GetPeerEndpoints helpers to derive
  peers' listen endpoints (outbound: dialed port; inbound: advertised),
  with IPv4-mapped-IPv6 normalization and IP+port dedup
- Match ping pong/timeout callbacks by peer address (the UDP layer owns
  the nonce), fixing discovered peers stuck UNREACHABLE
- Ping the peer's listen port instead of the ephemeral TCP source port
  (fixes the original stub so pongs actually return)
- Add `peers` CLI command to dump the discovery table (endpoint/hop/
  state/ping)
- Add discovery tunables to constants.h (fanout, max hops, target
  connections, timeouts, caps)

include/tcpd/tcpconnection.h

@@ -26,6 +26,10 @@ struct tcp_connection_t {
     uint32_t connectionId;
     tcp_connection_role_t role;
 
+    // Peer's advertised TCP/UDP listen port (learned from HELLO/ACK_HELLO). 0 until known.
+    // For OUTBOUND connections the peerAddr port already is the listen port; this matters for INBOUND peers.
+    uint16_t peerListenPort;
+
     pthread_t ioThread;
     pthread_mutex_t sendLock;
     pthread_mutex_t stateLock;