test: hostile-input tests for the remote-facing parsers

Seeded pseudo-random input, biased towards each parser's control bytes,
is fed to the IRC line parser and state machine (plus mIRC formatting),
Telnet IAC negotiation, VNC framebuffer updates (including ZRLE behind a
real zlib stream, with a check that the decoders are actually reached),
~/.ssh/config parsing and the update manifest. None panic, and Telnet's
pending buffer stays bounded.

These run on stable in every CI run; they are not coverage-guided, so a
cargo-fuzz setup (nightly) remains a possible follow-up.

rust/core/src/clients/irc.rs

@@ -1052,6 +1052,23 @@ mod tests {
     use std::io::{BufRead, BufReader};
     use std::net::TcpListener;
 
+    #[test]
+    fn hostile_lines_never_panic() {
+        let client = Client::new(Config::default());
+        client.st().nick = "me".into();
+        client.handle_message(&parse_line(":me!u@h JOIN #a").unwrap());
+        let mut n = crate::Noise::new(0x1bc);
+        for _ in 0..5000 {
+            let raw = n.bytes(120, b":!@# ,;=\x01\x02\x03\x0f\x16\x1d\x1fme#aJOINPARTKICKPRIVMSGNOTICE3534330011234567890");
+            let line = String::from_utf8_lossy(&raw);
+            if let Some(m) = parse_line(&line) {
+                client.handle_message(&m);
+                let e = Event { kind: "message".into(), target: m.param(0).into(), from: m.source.clone(), text: m.last().into(), ..Default::default() };
+                let _ = crate::ircformat::format_event(&e, "12:00");
+            }
+        }
+    }
+
     #[test]
     fn remember_room_membership_tracks_confirmed_channels() {
         let updates = Arc::new(Mutex::new(Vec::new()));