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>

tests/config.rs

@@ -0,0 +1,55 @@
+//! Integration tests for config layering and `[network]` sections, exercising
+//! the public `rustbot::config::parse_str` (no filesystem or environment).
+
+use rustbot::config::parse_str;
+
+#[test]
+fn flat_config_is_a_single_network() {
+    let cfgs = parse_str("server = irc.example.org\nnick = flatbot\nchannels = #a #b\n");
+    assert_eq!(cfgs.len(), 1);
+    assert_eq!(cfgs[0].server, "irc.example.org");
+    assert_eq!(cfgs[0].nick, "flatbot");
+    assert_eq!(cfgs[0].channels, vec!["#a", "#b"]);
+    // The label defaults to the server host.
+    assert_eq!(cfgs[0].name, "irc.example.org");
+}
+
+#[test]
+fn sections_inherit_top_level_defaults_and_override() {
+    let cfgs = parse_str(
+        "nick = shared\nchannels = #lobby\n\
+         [libera]\nserver = irc.libera.chat\ntls = true\n\
+         [oftc]\nserver = irc.oftc.net\nnick = other\nchannels = #oftc\n",
+    );
+    assert_eq!(cfgs.len(), 2);
+
+    let libera = &cfgs[0];
+    assert_eq!(libera.name, "libera");
+    assert_eq!(libera.server, "irc.libera.chat");
+    assert_eq!(libera.nick, "shared"); // inherited
+    assert_eq!(libera.channels, vec!["#lobby"]); // inherited
+    assert!(libera.tls);
+    assert_eq!(libera.port, 6697); // TLS-port convention applied per network
+
+    let oftc = &cfgs[1];
+    assert_eq!(oftc.name, "oftc");
+    assert_eq!(oftc.server, "irc.oftc.net");
+    assert_eq!(oftc.nick, "other"); // overridden
+    assert_eq!(oftc.channels, vec!["#oftc"]); // overridden
+    assert!(!oftc.tls); // default
+}
+
+#[test]
+fn a_section_may_rename_itself() {
+    let cfgs = parse_str("[net]\nname = pretty\nserver = irc.example.org\n");
+    assert_eq!(cfgs.len(), 1);
+    assert_eq!(cfgs[0].name, "pretty");
+}
+
+#[test]
+fn comments_and_blanks_are_ignored() {
+    // A lone commented/blank file yields the built-in default single network.
+    let cfgs = parse_str("# comment\n; also\n\n   \n");
+    assert_eq!(cfgs.len(), 1);
+    assert_eq!(cfgs[0].server, "irc.tchatou.fr"); // built-in default
+}