feat(listener): ListenerQUIC::Port() reads back the port it bound

Port 0 asks msquic for an ephemeral port, but nothing read that port
back, so a QUIC server had to pick a fixed one and hope nothing else held
it, and this suite has to hand every QUIC test a port of its own.
ListenerStart writes the port of the UDP socket it bound into the
listener's local address; the constructor now reads it with
QUIC_PARAM_LISTENER_LOCAL_ADDRESS right after the start, and Port()
returns it. It is valid as soon as the constructor returns and still
after Stop(), and 0 on a moved-from listener. ListenerTCP has had the
same since d6f8f4e.

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

implementations/Crafter.Network-ListenerQUIC.cpp

@@ -150,6 +150,10 @@ struct ListenerQUIC::Impl {
     HQUIC listener = nullptr;
     HQUIC configuration = nullptr;
     std::vector<std::uint8_t> alpnBuf;
+    // The port msquic bound, read back once ListenerStart returned: a port
+    // of 0 asks msquic for an ephemeral one, and only the listener knows
+    // which it got.
+    std::uint16_t port = 0;
 
     ListenerQUIC* outer = nullptr;
 
@@ -301,6 +305,27 @@ ListenerQUIC::ListenerQUIC(std::uint16_t port,
         Runtime().api->ConfigurationClose(impl->configuration);
         throw QUICException(std::format("ListenerStart failed: 0x{:x}", static_cast<unsigned>(s)));
     }
+
+    // ListenerStart writes the port of the UDP socket it bound into the
+    // listener's local address when it was asked for port 0, and only
+    // then; read it back rather than trusting `port`. The UNSPEC address
+    // still carries it: the port sits at the same offset in sockaddr_in
+    // and sockaddr_in6, and QuicAddrGetPort reads the latter for anything
+    // that is not INET.
+    QUIC_ADDR bound{};
+    std::uint32_t boundSize = sizeof(bound);
+    s = Runtime().api->GetParam(impl->listener, QUIC_PARAM_LISTENER_LOCAL_ADDRESS, &boundSize, &bound);
+    if (QUIC_FAILED(s)) {
+        Runtime().api->ListenerClose(impl->listener);
+        Runtime().api->ConfigurationClose(impl->configuration);
+        throw QUICException(std::format("GetParam(QUIC_PARAM_LISTENER_LOCAL_ADDRESS) failed: 0x{:x}",
+                                        static_cast<unsigned>(s)));
+    }
+    impl->port = QuicAddrGetPort(&bound);
+}
+
+std::uint16_t ListenerQUIC::Port() const {
+    return impl ? impl->port : 0;
 }
 
 ListenerQUIC::ListenerQUIC(ListenerQUIC&& other) noexcept

interfaces/Crafter.Network-ListenerQUIC.cppm

@@ -38,6 +38,12 @@ namespace Crafter {
         ListenerQUIC(const ListenerQUIC&) = delete;
         ListenerQUIC(ListenerQUIC&&) noexcept;
 
+        // The UDP port the listener is bound to: the one asked for, or the
+        // one the system picked when that was 0. Read from msquic once the
+        // listener has started, so it is valid as soon as the constructor
+        // returns, and still after Stop(). 0 on a moved-from listener.
+        std::uint16_t Port() const;
+
         void Stop();
 
         // Block on this thread, dispatch each accepted connection on this