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/udpd/udpnode.h

@@ -0,0 +1,56 @@
+#ifndef UDP_NODE_H
+#define UDP_NODE_H
+
+#include <stdint.h>
+#include <stdbool.h>
+#include <pthread.h>
+#include <netinet/in.h>
+
+#include <udpd/udppackettype.h>
+
+#define UDP_LISTEN_PORT 9393
+#define UDP_PING_RETRY_INTERVAL_MS 1000
+#define UDP_PING_MAX_RETRIES 3
+#define UDP_MAX_PENDING_PINGS 64
+
+typedef struct {
+    uint64_t nonce;
+    struct sockaddr_storage dest;
+    uint64_t lastSentMs;
+    int retries;
+    bool active;
+} pending_ping_t;
+
+typedef struct udp_node {
+    int sockFd; // AF_INET6, IPV6_V6ONLY=1
+    int sockFdV4; // AF_INET
+
+    volatile int isRunning;
+
+    pthread_t recvThreadV6;
+    pthread_t recvThreadV4;
+    pthread_t retryThread;
+
+    pending_ping_t pendingPings[UDP_MAX_PENDING_PINGS];
+    pthread_mutex_t pingsMutex;
+
+    void (*on_pong)(struct udp_node* node,
+                    const struct sockaddr_storage* from,
+                    uint64_t nonce, int protoVersion, uint64_t rttMs, void* user);
+    void (*on_ping_timeout)(struct udp_node* node,
+                            const struct sockaddr_storage* dest,
+                            uint64_t nonce, void* user);
+    void* callbackUser;
+} udp_node_t;
+
+int UdpNode_Init(udp_node_t* node, uint16_t port);
+void UdpNode_SetCallbacks(udp_node_t* node,
+        void (*on_pong)(udp_node_t*, const struct sockaddr_storage*, uint64_t, int, uint64_t, void*),
+        void (*on_ping_timeout)(udp_node_t*, const struct sockaddr_storage*, uint64_t, void*),
+        void* user);
+int UdpNode_Start(udp_node_t* node);
+void UdpNode_Stop(udp_node_t* node);
+void UdpNode_Destroy(udp_node_t* node);
+int UdpNode_SendPing(udp_node_t* node, const struct sockaddr_storage* dest);
+
+#endif

include/udpd/udppackettype.h

@@ -0,0 +1,14 @@
+#ifndef UDP_PACKET_TYPE_H
+#define UDP_PACKET_TYPE_H
+
+typedef enum {
+    UDP_PACKET_TYPE_NONE = 0,
+    UDP_PACKET_TYPE_PING = 1,
+    UDP_PACKET_TYPE_PONG = 2,
+} udp_packet_type_t;
+
+// Wire sizes in bytes
+#define UDP_PING_WIRE_SIZE 9   // 1 (type) + 8 (nonce)
+#define UDP_PONG_WIRE_SIZE 13  // 1 (type) + 8 (nonce) + 4 (proto_version)
+
+#endif