stcp: ed25519 pubkey exchange in handshake, fix instance destroy segfault, fix chat_tcp_test

- u_async.c: process immediate_queue BEFORE epoll_wait in uasync_poll
- stcp_link.c: defensive NULL etcp_conn/etcp_link in stcp_link_close
- utun_instance.c: move stcp_server_list_destroy_all to Phase J (before ETCP),
  add deferred drain in both Phase J and L
- stcp.h/c: STCP_HS_ENC_CLIENT 6→38, STCP_HS_ENC_SERVER 7→39,
  exchange ed25519_pubkey in both handshake directions
- etcp_connections.c: copy peer_ed25519 from STCP to ETCP_CONN in
  etcp_link_enter_ready_tcp; guard link->etcp in tcp_link_close_cb
- chat_sync.c: fix lk->conn NULL dereference for TCP links
- test_stcp.c: update stcp_server_create/stcp_client_connect callsites
- tools/chat_tcp_test: change transport udp→tcp

src/ntp_node_time.h

@@ -0,0 +1,36 @@
+#ifndef NTP_NODE_TIME_H
+#define NTP_NODE_TIME_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+struct UTUN_INSTANCE;
+
+#define NTP_NODE_MAX_PEERS 32
+
+#define NTP_NODE_DRIFT_WARN_US  2000000LL   // 2 seconds
+#define NTP_NODE_DRIFT_ERROR_US 10000000LL  // 10 seconds
+
+struct NTP_NODE_PEER {
+    uint64_t node_id;
+    int64_t  offset_us;   // my_time_recv - peer_time (positive = my clock ahead)
+    int      has_offset;  // first offset already stored, subsequent deltas are real drift
+};
+
+struct NTP_NODE_TIME {
+    struct NTP_NODE_PEER peers[NTP_NODE_MAX_PEERS];
+    int peer_count;
+};
+
+int  ntp_node_time_init(struct UTUN_INSTANCE* inst);
+void ntp_node_time_destroy(struct UTUN_INSTANCE* inst);
+void ntp_node_sync_peers(struct UTUN_INSTANCE* inst);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // NTP_NODE_TIME_H