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.
