stcp: close old TCP links on reconnect, check stcp_link_is_ready before send, notify ETCP on STCP close

1a: tcp_server_on_link — close existing TCP links before creating new ETCP_LINK
1b: etcp_tcp_send — skip send if stcp_link not ready
3a+3b+3c: stcp_conn_set_on_close → stcp_link_on_stcp_close → marks link DOWN → triggers reconnect

src/transport_layer/etcp_connections.c

@@ -62,6 +62,9 @@ static void tcp_server_on_link(struct stcp_link *link, void *arg) {
         etcp_update_log_name(conn);
         { struct conn_queue_entry* ce = (struct conn_queue_entry*)conn->conn_queue_entry->data; ce->peer_node_id = node_id; queue_remove_data(conn->conn_queue, conn->conn_queue_entry); queue_data_put_with_index(conn->conn_queue, conn->conn_queue_entry); }
         DEBUG_INFO(DEBUG_CATEGORY_ETCP, "tcp_server_on_link: new ETCP_CONN peer=0x%016llx", (unsigned long long)node_id);
+    } else {
+        struct ETCP_LINK* nl = conn->links;
+        while (nl) { struct ETCP_LINK* nx = nl->next; if (nl->is_tcp) etcp_link_close(nl); nl = nx; }
     }
 
     struct ETCP_LINK *tlink = etcp_link_new(conn, NULL, NULL, 1);
@@ -1127,6 +1130,7 @@ static void tcp_link_close_cb(struct stcp_link *sl, int err, void *arg) {
 
 static int etcp_tcp_send(struct ETCP_DGRAM* dgram) {
     struct ETCP_LINK *link = dgram->link;
+    if (!link->tcp_link || !stcp_link_is_ready(link->tcp_link)) return -1;
     link->pkt_sent_since_keepalive = 1;
     dgram->flag_up = link->recv_keepalive;
     int rc;

src/transport_layer/stcp_link.c

@@ -78,6 +78,18 @@ static void link_rx_cb(struct ll_queue *q, void *arg) {
     queue_resume_callback(q);
 }
 
+// ====== STCP close → link down ======
+
+#include "etcp_connections.h"
+
+static void stcp_link_on_stcp_close(struct stcp_conn *conn, int err, void *arg) {
+    struct stcp_link *link = (struct stcp_link *)arg;
+    (void)conn; (void)err;
+    link->conn = NULL; link->ready = 0;
+    if (link->etcp_link) { link->etcp_link->recv_keepalive = 0; link->etcp_link->link_status = 0; }
+    if (link->on_close_cb) link->on_close_cb(link, err, link->close_arg);
+}
+
 // ====== server accept → link ======
 
 static void server_accept_cb(struct stcp_conn *conn, void *arg) {
@@ -88,6 +100,8 @@ static void server_accept_cb(struct stcp_conn *conn, void *arg) {
     link->ready = 1;
     link->conn = conn;
 
+    stcp_conn_set_on_close(conn, stcp_link_on_stcp_close, link);
+
     struct ll_queue *rx = queue_new(conn->ua, 0, 0, 0, "srx");
     queue_set_callback(rx, link_rx_cb, link);
     stcp_conn_set_rx_queue(conn, rx);
@@ -108,6 +122,8 @@ static void client_ready_cb(struct stcp_conn *conn, void *arg) {
     link->ready = 1;
     link->conn = conn;
 
+    stcp_conn_set_on_close(conn, stcp_link_on_stcp_close, link);
+
     struct ll_queue *rx = queue_new(conn->ua, 0, 0, 0, "crx");
     queue_set_callback(rx, link_rx_cb, link);
     stcp_conn_set_rx_queue(conn, rx);