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/zxing-cpp/core/src/TextDecoder.cpp

@@ -0,0 +1,54 @@
+/*
+* Copyright 2016 Nu-book Inc.
+* Copyright 2022 gitlost
+* Copyright 2025 Axel Waggershauser
+*/
+// SPDX-License-Identifier: Apache-2.0
+
+#include "TextDecoder.h"
+
+#include "ZXAlgorithms.h"
+#include "libzueci/zueci.h"
+
+#include <cassert>
+#include <stdexcept>
+#include <cstdio>
+
+namespace ZXing {
+
+std::string BytesToUtf8(ByteView bytes, ECI eci)
+{
+	constexpr unsigned int replacement = 0xFFFD;
+	constexpr unsigned int flags = ZUECI_FLAG_SB_STRAIGHT_THRU | ZUECI_FLAG_SJIS_STRAIGHT_THRU;
+	int utf8_len;
+
+	if (eci == ECI::Unknown)
+		eci = ECI::Binary;
+
+	int error_number = zueci_dest_len_utf8(ToInt(eci), bytes.data(), Size(bytes), replacement, flags, &utf8_len);
+	if (error_number >= ZUECI_ERROR) {
+#ifdef PRINT_DEBUG
+		fprintf(stderr, "zueci_dest_len_utf8 error: %d\n", error_number);
+#endif
+		// throw FormatError("Invalid UTF-8 data");
+		return {};
+	}
+
+	std::string utf8(utf8_len, 0);
+
+	error_number = zueci_eci_to_utf8(ToInt(eci), bytes.data(), Size(bytes), replacement, flags,
+									 reinterpret_cast<uint8_t*>(utf8.data()), &utf8_len);
+	if (error_number >= ZUECI_ERROR) {
+#ifdef PRINT_DEBUG
+		fprintf(stderr, "zueci_eci_to_utf8 error: %d\n", error_number);
+#endif
+		// throw FormatError("Invalid UTF-8 data");
+		return {};
+	}
+
+	assert(Size(utf8) == utf8_len);
+
+	return utf8;
+}
+
+} // ZXing