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.

tests/protocol.rs

@@ -0,0 +1,80 @@
+//! Integration tests for the IRC protocol layer, exercising the public
+//! `rustbot::irc` API: message parsing, IRCv3 tags, base64, and server-time.
+
+use rustbot::irc::{base64_encode, parse_server_time, Message};
+
+#[test]
+fn parses_privmsg_with_prefix() {
+    let m = Message::parse(":alice!a@host PRIVMSG #chan :hi there\r\n").unwrap();
+    assert_eq!(m.command, "PRIVMSG");
+    assert_eq!(m.nick(), Some("alice"));
+    assert_eq!(m.param(0), Some("#chan"));
+    assert_eq!(m.trailing(), Some("hi there"));
+}
+
+#[test]
+fn parses_ping_without_prefix() {
+    let m = Message::parse("PING :LAG1234").unwrap();
+    assert_eq!(m.command, "PING");
+    assert!(m.prefix.is_none());
+    assert_eq!(m.trailing(), Some("LAG1234"));
+}
+
+#[test]
+fn parses_and_unescapes_tags() {
+    let m = Message::parse("@id=123;key=a\\sb\\:c :n!u@h PRIVMSG #c :yo").unwrap();
+    assert_eq!(m.tag("id"), Some("123"));
+    assert_eq!(m.tag("key"), Some("a b;c"));
+    assert_eq!(m.command, "PRIVMSG");
+    assert_eq!(m.trailing(), Some("yo"));
+}
+
+#[test]
+fn parses_numeric_reply() {
+    let m = Message::parse(":srv 001 me :Welcome to the network").unwrap();
+    assert_eq!(m.command, "001");
+    assert_eq!(m.param(0), Some("me"));
+    assert_eq!(m.trailing(), Some("Welcome to the network"));
+}
+
+#[test]
+fn full_prefix_is_split() {
+    // Prefix parsing is exercised through the public `Message::parse`.
+    let m = Message::parse(":nick!user@host.example PRIVMSG #c :hi").unwrap();
+    let p = m.prefix.expect("prefix present");
+    assert_eq!(p.nick.as_deref(), Some("nick"));
+    assert_eq!(p.user.as_deref(), Some("user"));
+    assert_eq!(p.host.as_deref(), Some("host.example"));
+    assert_eq!(p.name(), "nick");
+}
+
+#[test]
+fn blank_line_is_none() {
+    assert!(Message::parse("   \r\n").is_none());
+}
+
+#[test]
+fn base64_matches_known_vectors() {
+    assert_eq!(base64_encode(b""), "");
+    assert_eq!(base64_encode(b"f"), "Zg==");
+    assert_eq!(base64_encode(b"fo"), "Zm8=");
+    assert_eq!(base64_encode(b"foobar"), "Zm9vYmFy");
+    // A SASL PLAIN payload: authzid="" authcid="me" passwd="pw"
+    assert_eq!(base64_encode(b"\0me\0pw"), "AG1lAHB3");
+}
+
+#[test]
+fn server_time_parses_to_unix_seconds() {
+    assert_eq!(parse_server_time("1970-01-01T00:00:00.000Z"), Some(0));
+    assert_eq!(parse_server_time("2020-01-01T00:00:00Z"), Some(1_577_836_800));
+    assert_eq!(parse_server_time("2021-01-01T00:00:00.123Z"), Some(1_609_459_200));
+    assert_eq!(parse_server_time("not-a-timestamp"), None);
+}
+
+#[test]
+fn reads_message_tags_via_accessor() {
+    let m = Message::parse("@time=2021-01-01T00:00:00Z;batch=42 :s PRIVMSG #c :hi").unwrap();
+    assert_eq!(m.tag("batch"), Some("42"));
+    assert_eq!(m.tag("time"), Some("2021-01-01T00:00:00Z"));
+    assert_eq!(m.tag("missing"), None);
+}