fix: prevent use-after-free in etcp_ack_recv when link closed with pending inflight

When etcp_link_close() frees a link while INFLIGHT_PACKET entries
still reference it via last_link, a later ACK causes SIGSEGV in
bbr_main(link->bbr=NULL).

Fix:
- etcp_inflight_nullify_link(): nullifies last_link in both
  input_wait_ack and input_send_q for entries pointing to dead_link
- Called from etcp_link_close() before u_free(link)
- Guard etcp_ack_recv: check acked_pkt->last_link->bbr before
  accessing BBR/inflight stats
- Also fix bbr leak in etcp_link_close !link->conn path

Verified: 30/30 runs of test_etcp_link_stress pass (was ~50% crash)
Full suite: 71/71 pass

src/transport_layer/etcp.c

@@ -1410,7 +1410,7 @@ void etcp_ack_recv(struct ETCP_CONN* etcp, uint32_t seq, uint16_t ts, uint16_t d
     }
 
     // === subtract inflight from the LAST link the packet was sent on ===
-    if (acked_pkt->last_link) {
+    if (acked_pkt->last_link && acked_pkt->last_link->bbr) {
             struct ETCP_LINK* link = acked_pkt->last_link;
             link->inflight_bytes -= acked_pkt->ll.len;
             link->inflight_packets--;

src/transport_layer/etcp_connections.c

@@ -1074,6 +1074,18 @@ void etcp_link_update_inflight_lim(struct ETCP_LINK* link, uint32_t new_lim) {
     etcp_conn_on_inflight_lim_changed(link->etcp);
 }
 
+// Обнуляет last_link для всех inflight-записей ссылающихся на dead_link.
+// Вызывается перед u_free(link) чтобы etcp_ack_recv не упал на use-after-free.
+static void etcp_inflight_nullify_link(struct ll_queue* q, struct ETCP_LINK* dead_link) {
+    if (!q) return;
+    struct ll_entry* entry = q->head;
+    while (entry) {
+        struct INFLIGHT_PACKET* inf = (struct INFLIGHT_PACKET*)entry->data;
+        if (inf->last_link == dead_link) inf->last_link = NULL;
+        entry = entry->next;
+    }
+}
+
 void etcp_link_close(struct ETCP_LINK* link) {
     DEBUG_TRACE(DEBUG_CATEGORY_CONNECTION, "");
     if (!link) return;
@@ -1096,6 +1108,7 @@ void etcp_link_close(struct ETCP_LINK* link) {
         if (link->shaper_timer) uasync_cancel_timeout(link->etcp->instance->ua, link->shaper_timer);
         if (link->keepalive_timer) uasync_cancel_timeout(link->etcp->instance->ua, link->keepalive_timer);
         etcp_conn_on_inflight_lim_changed(link->etcp);
+        u_free(link->bbr);
         u_free(link);
         return;
     }
@@ -1136,6 +1149,8 @@ void etcp_link_close(struct ETCP_LINK* link) {
                link->etcp->log_name, link->local_link_id, sockaddr_storage_to_str(&link->remote_addr).str, link->total_decrypted,
                (unsigned long long)link->acked_bytes, link->inflight_bytes, link->inflight_packets);
     etcp_fire_link_status_cbk(link, link->link_state, link->link_status);
+    etcp_inflight_nullify_link(link->etcp->input_wait_ack, link);
+    etcp_inflight_nullify_link(link->etcp->input_send_q, link);
     u_free(link->bbr);
     u_free(link);
 }