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)

tools/chatgui/src/qrcode_utils.cpp

@@ -1,26 +0,0 @@
-#include "qrcode_utils.h"
-#include "ZXingQt.h"
-
-QImage generateQRCode(const QString& text, int moduleSize)
-{
-    using namespace ZXingQt;
-    auto barcode = Barcode::fromText(text, BarcodeFormat::QRCode);
-    if (!barcode.isValid())
-        return {};
-    WriterOptions opts;
-    if (moduleSize > 0)
-        opts.scale(moduleSize);
-    return barcode.toImage(opts);
-}
-
-QStringList readQRCodes(const QImage& image)
-{
-    using namespace ZXingQt;
-    auto results = ReadBarcodes(image);
-    QStringList texts;
-    for (const auto& b : results) {
-        if (b.isValid())
-            texts.append(b.text());
-    }
-    return texts;
-}