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/lib.rs

@@ -0,0 +1,23 @@
+//! rustbot — a small, modular IRC bot, split into a reusable library (this
+//! crate) and a thin binary (`src/main.rs`).
+//!
+//! Modules:
+//!   * [`irc`]    — protocol + transport (the "IRC library"): message parsing,
+//!                  IRCv3 tags, and a blocking TCP/TLS [`irc::Connection`].
+//!   * [`bot`]    — behavior: registration, capability negotiation, the event
+//!                  loop, and commands, driving one [`irc::Connection`].
+//!   * [`config`] — the layered [`config::Config`] and how it is loaded from a
+//!                  file (with optional `[network]` sections) and environment.
+//!
+//! The binary adds only a per-network connect/reconnect supervisor on top, so
+//! the whole bot is exercisable — and reusable — through this library's API.
+
+pub mod bot;
+pub mod config;
+pub mod irc;
+
+/// Tiny stderr logger shared by the library and the binary. Swap for the
+/// `log`/`tracing` crates if you outgrow it.
+pub fn log(msg: &str) {
+    eprintln!("[rustbot] {msg}");
+}