feat: SASL authentication and IRCv3 capability negotiation

[chrj]

What this changes

The registration handshake was a bare NICK/USER pair, so a bot could not identify itself to services. Most networks need this: +r channels refuse an unauthenticated bot, and the account cloak never applies.

Registration now sends CAP LS 302 before NICK/USER, which makes the server hold registration open, and closes the exchange with CAP END. SASL runs inside that window.

Server::tls("irc.libera.chat:6697").with_sasl_plain("mybot", &password)
Server::tls("…").with_client_cert_pem(pem).with_sasl_external()   // CertFP
Server::plain("…").with_password("…")                              // PASS
Server::plain("…").with_capabilities(["server-time"])

The builders are on Server and on TlsServer.

Why the credentials live on Server

The generated new() connects immediately. By the time a State exists for a with_* builder to configure, the handshake is over. Server is the only value the caller supplies early enough, and it already travels into Blueprint, so a reconnect re-authenticates for free.

A refused SASL exchange fails the connection

If the network offers no sasl capability, refuses the mechanism, or rejects the credentials, connect returns an error. The bot does not continue unauthenticated. Arriving without your identity is the fault SASL prevents, so it is loud rather than silent.

A bot with no credentials sends the same handshake as before, and no CAP line at all.

Two failures this had to avoid

A dropped nick collision. The server answers NICK as soon as it reads it, which is in the middle of the capability exchange. The handshake buffers every line that does not belong to it and hands them back; the read loop drains those before it touches the socket. Without this the fallback nick would have stopped working.

A downgrade after a hot reload. A reloaded process rebuilt its Server from the inherited environment, which holds only the address. The inherited socket is already registered, so the bot kept working, but a later reconnect re-ran the handshake with no credentials. The generated new() now keeps the Server the caller passed. Nothing secret goes through the exec environment.

Notes

• No new dependency. base64 is about 25 lines in auth.rs, tested against the RFC 4648 vectors. The test-util feature is added to the existing tokio dev-dependency so the timeout test runs on a paused clock.
• PASS and AUTHENTICATE log a redacted form, and Debug redacts every credential, so a password cannot reach the protocol trace. Tests pin this.
• The capability exchange is bounded by a 30 second timeout and by a line count, so a server that streams instead of answering cannot stall the bot or grow the buffer without limit.
• All additive. No signature changed.

Testing

18 tests in ircbot/tests/sasl.rs drive the real State::connect against a scripted server on loopback and assert the bytes on the wire: the RFC 4616 payload for PLAIN, the empty response for EXTERNAL, multi-line CAP LS reassembly, a PING during the exchange, a nick collision reaching the read loop, and the message of every failure mode. 9 unit tests cover base64, redaction, capability selection, and CAP line parsing.

cargo test --workspace, cargo clippy --workspace --all-targets --features tls -- -D warnings, cargo fmt --all --check, and cargo doc with -D warnings all pass.