feat(tcp): connect over IPv4 and IPv6, and say why a connect failed

- ClientTCP resolves with AF_UNSPEC and tries every address the name
  resolves to, in getaddrinfo's order, the way Python's
  socket.create_connection does. It used to ask for AF_INET only and keep
  the first answer, so `localhost` could not reach a server listening on
  ::1 alone, which is what a dev server on a recent Node does. The connect
  timeout bounds each address on its own; when none connects, what the
  last one failed with is thrown.
- A failed resolve, socket() or connect throws SocketErrorException: the
  std::runtime_error it always was, with the same message, plus the errno
  (SocketErrorKind::System) or the getaddrinfo code
  (SocketErrorKind::Resolver). A caller that reports the cause in its own
  words, such as a Python-style "[Errno 111] Connection refused", no
  longer has to parse it back out of the text.
- The move constructor carries the resolved addresses along.

New: ShouldConnectDualStack, ShouldReportConnectErrors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/ShouldConnectDualStack/main.cpp

@@ -0,0 +1,151 @@
+//SPDX-License-Identifier: LGPL-3.0-only
+//SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®
+
+// ClientTCP resolves a host for IPv4 and IPv6 alike and tries every address
+// the name resolves to, in order, the way Python's socket.create_connection
+// does. Before, it asked getaddrinfo for AF_INET only and kept the first
+// answer, so a server listening on ::1 alone — a dev server on a recent Node
+// binds `localhost` that way — could not be reached by name, while every
+// Python client reached it.
+//
+// The listeners are raw sockets, so nothing here depends on ListenerTCP: one
+// IPv6-only on ::1, one on 127.0.0.1. Skips (77) on a host without IPv6
+// loopback.
+
+#include <arpa/inet.h>
+#include <netdb.h>
+#include <netinet/in.h>
+#include <sys/socket.h>
+#include <unistd.h>
+#include <cerrno>
+#include <cstdlib>
+
+import Crafter.Network;
+import std;
+using namespace Crafter;
+using namespace std::chrono_literals;
+
+namespace {
+
+int failures = 0;
+
+void Check(bool ok, std::string_view what) {
+    std::println("{}  {}", ok ? "PASS" : "FAIL", what);
+    if (!ok) ++failures;
+}
+
+// A socket listening on the loopback address of `family`, on a port the
+// kernel picks; fd stays -1 when the host cannot open one.
+struct LoopbackListener {
+    int fd = -1;
+    std::uint16_t port = 0;
+
+    explicit LoopbackListener(int family) {
+        fd = socket(family, SOCK_STREAM, 0);
+        if (fd == -1) return;
+        sockaddr_storage address {};
+        socklen_t length = 0;
+        if (family == AF_INET6) {
+            // v6 only, or the socket would also take IPv4 connects and the
+            // checks below would not tell the two apart.
+            int only = 1;
+            setsockopt(fd, IPPROTO_IPV6, IPV6_V6ONLY, &only, sizeof(only));
+            auto* v6 = reinterpret_cast<sockaddr_in6*>(&address);
+            v6->sin6_family = AF_INET6;
+            v6->sin6_addr = in6addr_loopback;
+            length = sizeof(sockaddr_in6);
+        } else {
+            auto* v4 = reinterpret_cast<sockaddr_in*>(&address);
+            v4->sin_family = AF_INET;
+            v4->sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+            length = sizeof(sockaddr_in);
+        }
+        if (bind(fd, reinterpret_cast<sockaddr*>(&address), length) != 0 || listen(fd, 8) != 0) {
+            close(fd);
+            fd = -1;
+            return;
+        }
+        getsockname(fd, reinterpret_cast<sockaddr*>(&address), &length);
+        port = family == AF_INET6 ? ntohs(reinterpret_cast<sockaddr_in6*>(&address)->sin6_port)
+                                  : ntohs(reinterpret_cast<sockaddr_in*>(&address)->sin_port);
+    }
+    ~LoopbackListener() {
+        if (fd != -1) close(fd);
+    }
+};
+
+// The address family of the peer a connected client reached.
+int PeerFamily(const ClientTCP& client) {
+    sockaddr_storage address {};
+    socklen_t length = sizeof(address);
+    getpeername(client.socketid, reinterpret_cast<sockaddr*>(&address), &length);
+    return address.ss_family;
+}
+
+// Whether `localhost` resolves to ::1 here; /etc/hosts decides.
+bool LocalhostResolvesToIPv6() {
+    addrinfo hints {};
+    hints.ai_family = AF_INET6;
+    hints.ai_socktype = SOCK_STREAM;
+    addrinfo* resolved = nullptr;
+    if (getaddrinfo("localhost", nullptr, &hints, &resolved) != 0) return false;
+    freeaddrinfo(resolved);
+    return true;
+}
+
+} // namespace
+
+int main() {
+    LoopbackListener v6(AF_INET6);
+    if (v6.fd == -1) {
+        std::println("SKIP  no IPv6 loopback on this host");
+        return 77;
+    }
+
+    {
+        ClientTCP client("::1", v6.port, 2s);
+        Check(PeerFamily(client) == AF_INET6, "an IPv6 literal connects over IPv6");
+    }
+
+    if (LocalhostResolvesToIPv6()) {
+        try {
+            ClientTCP client("localhost", v6.port, 2s);
+            Check(PeerFamily(client) == AF_INET6, "localhost reaches a server that listens on ::1 only");
+        } catch (const std::exception& e) {
+            Check(false, std::format("localhost reaches a server that listens on ::1 only (threw: {})", e.what()));
+        }
+    } else {
+        std::println("SKIP  localhost does not resolve to ::1 on this host");
+    }
+
+    {
+        // Whichever address the resolver ranks first, a refused one is not
+        // the end: the next is tried.
+        LoopbackListener v4(AF_INET);
+        try {
+            ClientTCP client("localhost", v4.port, 2s);
+            Check(PeerFamily(client) == AF_INET, "localhost reaches a server that listens on 127.0.0.1 only");
+        } catch (const std::exception& e) {
+            Check(false, std::format("localhost reaches a server that listens on 127.0.0.1 only (threw: {})", e.what()));
+        }
+    }
+
+    {
+        std::uint16_t closedPort = 0;
+        {
+            LoopbackListener gone(AF_INET);
+            closedPort = gone.port;
+        }
+        bool refused = false;
+        try {
+            ClientTCP client("localhost", closedPort, 2s);
+        } catch (const SocketErrorException& e) {
+            refused = e.kind == SocketErrorKind::System && e.code == ECONNREFUSED;
+        } catch (const std::exception&) {
+        }
+        Check(refused, "when no address answers, the last failure is thrown, with its errno");
+    }
+
+    std::println("{}", failures == 0 ? "all checks passed" : "check(s) failed");
+    return failures == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
+}