Add TLS transport support

Connect over TLS (default port 6697) using native-tls with the system
OpenSSL trust store for certificate verification. Unify the plaintext and
TLS transports behind a single Box<dyn Read + Write>, dropping the
read/write socket clone that a TLS session cannot support. Add a `tls`
config key and IRC_TLS override; the port auto-defaults to 6697 when TLS
is enabled.

src/main.rs

@@ -44,7 +44,7 @@ fn main() {
 
 /// Run a single connection lifecycle: connect, register, loop until disconnect.
 fn run_once(config: &Config) -> std::io::Result<()> {
-    let conn = Connection::connect(&config.server, config.port)?;
+    let conn = Connection::connect(&config.server, config.port, config.tls)?;
     let mut bot = Bot::new(config.clone(), conn);
     bot.run()
 }
@@ -62,6 +62,12 @@ fn load_config() -> Config {
     }
     apply_env(&mut c);
 
+    // Convention: TLS listens on 6697. If TLS is enabled but the port is still
+    // the plaintext default, switch to the conventional TLS port.
+    if c.tls && c.port == 6667 {
+        c.port = 6697;
+    }
+
     c
 }
 
@@ -101,6 +107,7 @@ fn apply_config_file(c: &mut Config, path: &Path) -> io::Result<()> {
                 Ok(p) => c.port = p,
                 Err(_) => log(&format!("{}:{}: invalid port '{value}'", path.display(), i + 1)),
             },
+            "tls" => c.tls = parse_bool(value),
             "nick" => c.nick = value.to_string(),
             "user" => c.user = value.to_string(),
             "realname" => c.realname = value.to_string(),
@@ -127,6 +134,9 @@ fn apply_env(c: &mut Config) {
     if let Some(p) = std::env::var("IRC_PORT").ok().and_then(|v| v.parse().ok()) {
         c.port = p;
     }
+    if let Ok(v) = std::env::var("IRC_TLS") {
+        c.tls = parse_bool(&v);
+    }
     if let Ok(v) = std::env::var("IRC_NICK") {
         c.user = v.clone();
         c.nick = v;
@@ -151,6 +161,14 @@ fn apply_env(c: &mut Config) {
     }
 }
 
+/// Parse a boolean config value. Accepts `1`, `true`, `yes`, `on` (any case).
+fn parse_bool(value: &str) -> bool {
+    matches!(
+        value.trim().to_ascii_lowercase().as_str(),
+        "1" | "true" | "yes" | "on"
+    )
+}
+
 /// Split a channel list on commas and/or whitespace, dropping empties.
 fn split_list(value: &str) -> Vec<String> {
     value