Restructure into a library crate with modular bot and irc

Split the single-binary crate into a reusable `rustbot` library (src/lib.rs)
plus a thin supervisor binary. The monolithic irc.rs and bot.rs become module
folders:

  irc/    message (wire parsing), connection (TCP/TLS transport), encoding
  bot/    mod (state + event loop), caps (IRCv3 cap negotiation + SASL),
          commands (message routing + command table)
  config  Config + layered loading, moved out of main.rs and bot.rs

main.rs is now just the per-network connect/reconnect supervisor over the
library API. Unit tests move out of the source files into tests/ integration
tests (config.rs, protocol.rs) that drive the public surface — prefix parsing
is now covered through Message::parse, and config layering through the new
public config::parse_str. Behavior is unchanged; no new dependencies.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/irc/mod.rs

@@ -0,0 +1,24 @@
+//! Minimal IRC protocol layer — the "IRC library".
+//!
+//! Implements just enough of the modern IRC client protocol
+//! (<https://modern.ircdocs.horse/>) to drive a bot:
+//!
+//!   * [`Message`] / [`Prefix`] — parsing of the wire format
+//!     `@tags :source COMMAND params`, including IRCv3 message tags and
+//!     trailing parameters ([`message`]).
+//!   * [`Connection`] — a blocking TCP/TLS transport with convenience senders
+//!     ([`connection`]).
+//!   * Small self-contained helpers — base64 and `server-time` parsing
+//!     ([`encoding`]) — so the layer stays dependency-free apart from TLS.
+//!
+//! It is deliberately self-contained, so it can grow into a small reusable IRC
+//! library independent of the bot logic in [`crate::bot`].
+#![allow(dead_code)] // Library-style module; not every helper/sender is used yet.
+
+mod connection;
+mod encoding;
+mod message;
+
+pub use connection::Connection;
+pub use encoding::{base64_encode, now_unix, parse_server_time};
+pub use message::{Message, Prefix};