refactor: STCP connections as ETCP links (is_tcp)

- Add is_tcp, tcp_link, tcp_reconnect_* fields to ETCP_LINK
- Remove transport_link from ETCP_CONN, add tcp_link_count
- stcp_link: replace embedded etcp_conn with pointer; add etcp_link backpointer
- stcp_link: link_rx_cb routes ETCP data to etcp_conn_input
- stcp_link: call on_close_cb in close
- New TCP functions: etcp_link_enter_ready_tcp, etcp_tcp_link_start_connect,
  etcp_tcp_link_start_reconnect, tcp_link_close_cb, etcp_tcp_send
- etcp_link_new: handle NULL conn for TCP links; skip enter_init for TCP
- etcp_link_close: close stcp_link for TCP
- etcp_encrypt_send: redirect TCP to etcp_tcp_send (skips AES-CCM)
- tcp_server_on_link: create ETCP_LINK(is_tcp=1) + enter_ready_tcp
- etcp_connect: TCP addrs -> etcp_tcp_link_start_connect
- Remove ctx->tcp_link/tcp_ready
- test_stcp_traffic: find stcp_link via links->is_tcp instead of transport_link
- 66/68 tests pass; 2 STCP tests expected to fail (integration not complete)

tests/buffer_overflow_fix.patch

@@ -1,38 +0,0 @@
---- test_etcp_simple_traffic.c.backup
-+++ test_etcp_simple_traffic.c
-@@ -172,19 +172,25 @@
-     DEBUG_INFO(DEBUG_CATEGORY_ETCP, "SIMULATE RX: inst=%s, len=%u", 
-                 inst->is_server ? "server" : "client", len);
-     
--    // Create a mock ETCP_DGRAM for processing
--    struct ETCP_DGRAM mock_dgram;
--    memset(&mock_dgram, 0, sizeof(mock_dgram));
--    mock_dgram.link = NULL; // Will be set by connection processing
--    mock_dgram.data_len = len > 1500 ? 1500 : len;
--    mock_dgram.timestamp = get_current_timestamp();
--    if (mock_dgram.data_len > 0) {
--        memcpy(mock_dgram.data, data, mock_dgram.data_len);
-+    // Create a mock ETCP_DGRAM for processing with proper allocation
-+    size_t dgram_size = sizeof(struct ETCP_DGRAM) + (len > 1500 ? 1500 : len);
-+    struct ETCP_DGRAM* mock_dgram = malloc(dgram_size);
-+    if (!mock_dgram) {
-+        DEBUG_ERROR(DEBUG_CATEGORY_ETCP, "Failed to allocate mock dgram");
-+        return;
-+    }
-+    memset(mock_dgram, 0, dgram_size);
-+    mock_dgram->link = NULL; // Will be set by connection processing
-+    mock_dgram->data_len = len > 1500 ? 1500 : len;
-+    mock_dgram->timestamp = get_current_timestamp();
-+    if (mock_dgram->data_len > 0) {
-+        memcpy(mock_dgram->data, data, mock_dgram->data_len);
-     }
-     
-     // Process through ETCP input
--    etcp_conn_input(&mock_dgram);
-+    etcp_conn_input(mock_dgram);
-+    
-+    // Free the allocated memory
-+    free(mock_dgram);
- }
- 
- // Simulate packet transmission for analysis