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

tools/chatgui/src/qrcode_utils.cpp

@@ -0,0 +1,26 @@
+#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;
+}