docs(listener): ListenerQUIC::Port(), and port 0 for tests

The README gains a QUIC section, which it never had: plain QUIC under an
ALPN of your own, starting with a listener on port 0 and Port(). The
consumer notes describe Port() and point tests at port 0 rather than at
binding a socket by hand; the agent notes count the new test and make
port 0 the default for a new one.

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

README.md

@@ -85,6 +85,23 @@ std::thread([&] { broker.ListenSyncSync(); }).detach();
 // ... broker.Stop() ends the loop from another thread.
 ```
 
+### QUIC Components
+
+Plain QUIC under an ALPN of your own, for a protocol that is not HTTP. A browser cannot reach these: it speaks QUIC only as WebTransport, see [WebTransport Components](#webtransport-components).
+
+#### ListenerQUIC
+```cpp
+Crafter::QUICServerCredentials creds;
+creds.selfSigned = true; // dev-only, see "Self-signed certs & browser peers"
+Crafter::ListenerQUIC listener(0, "myproto/1", creds, [](Crafter::ClientQUIC* peer) {
+    // An accepted connection, heap-allocated and yours to delete.
+});
+listener.ListenAsyncAsync();
+const std::uint16_t port = listener.Port(); // the port msquic picked for port 0
+```
+
+`Port()` reports the UDP port the listener is bound to, so port 0 is usable: the listener takes whatever port is free and tells you which.
+
 ### HTTP/3 Components
 
 HTTP/3 runs over QUIC, which always requires TLS. Pass server credentials when constructing the listener (or set `selfSigned = true` for a development-only ephemeral cert) and matching client credentials when constructing the client (`insecureNoServerValidation = true` for self-signed servers).
@@ -316,6 +333,7 @@ The library includes tests covering:
 - HTTP/3 external interop (`ShouldSend`) — live fetch from `cloudflare-quic.com:443`, exercises real TLS chain validation, mandatory control stream, peer-initiated unidi streams, and QPACK Huffman decoding
 - QUIC reliable streams (`ShouldSendRecieveQUICStream`)
 - QUIC unreliable datagrams (`ShouldSendRecieveQUICDatagram`)
+- QUIC listener ports (`ShouldReportListenerPortQUIC`) — a listener on port 0 reports the port it got and is reachable there, a second one gets another, and a fixed port reads back as itself
 - QUIC connection teardown (`ShouldSurviveConnectionChurn`) — 25 build-up/tear-down cycles with reader threads parked inside `RecieveSync`, the shape a reconnect loop has; the rest of the QUIC tests end in `std::_Exit(0)` and so never run a `ClientQUIC` destructor
 - WebTransport echo (`ShouldEchoWebTransport`) — extended-CONNECT acceptance, draft-02 SETTINGS, bidi data stream framing (`WT_STREAM 0x41` + session-id varint), and byte-for-byte echo
 - HTTP/1.1 wire format (`ShouldParseHTTP1`) — serialisation, incremental parsing fed one byte at a time, chunked bodies with trailers, pipelining, interim 1xx, HTTP/1.0 and HEAD framing, and the malformed inputs the parser is required to reject