irc: split PRIVMSG on embedded newlines + widen host overhead

Two fixes to send_privmsg, both surfaced by correspondents reporting
truncated messages:

1. Multi-line content (code blocks, formatted text) sent as a single
   PRIVMSG was being truncated at the first '\n' by the IRC server —
   newlines are end-of-command markers. Split the message on newlines
   and send each line as its own PRIVMSG; skip empty lines since most
   servers reject empty PRIVMSGs.

2. Overhead computation assumed a host field of 63 bytes. OFTC's
   cloaked hostmasks can be longer, occasionally pushing the server-
   prepended prefix past 512 bytes and causing silent truncation.
   Raise the host budget to 80 and align the formula with the actual
   ':nick!~nick@host' prefix shape.

Also extended the word-boundary lookback from a fixed 10 chars to
max_msg / 4 — dense content (code) rarely had a space within 10 chars
of the length cap, so we were falling back to the char boundary and
splitting mid-word. Checking bytes[j-1] for a space (instead of
bytes[j]) drops leading whitespace from the rest-fragment.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

channels/irc/src/main.rs

@@ -237,11 +237,19 @@ impl State {
     async fn send_privmsg(&mut self, target: &str, msg: &str) -> io::Result<()> {
         // Send PRIVMSG, which is used for both private and channel messages.
         // Splits into multiple fragments if necessary.
-        // IRC max line = 512 bytes including CRLF. The server prepends
-        // our prefix when relaying: ":nick!~user@host PRIVMSG target :msg\r\n"
+        //
+        // Two constraints:
+        //   1. IRC max line = 512 bytes including CRLF. The server prepends
+        //      our prefix when relaying: ":nick!~user@host PRIVMSG target :msg\r\n"
+        //      So per-PRIVMSG message content must fit in 512 - overhead.
+        //   2. Embedded '\n' in the message would be interpreted by the
+        //      server as an end-of-command marker, truncating us. Split
+        //      on newlines first and send each line as its own PRIVMSG.
+        //
         // User is often ~nick (nick_len + 1). Host is up to 63 bytes.
+        // Cloaked OFTC hosts can be longer - pad the budget.
         let nick_len = self.config.nick.len();
-        let overhead = 1 + nick_len + 2 + nick_len + 1 + 63
+        let overhead = 1 + nick_len + 1 + (nick_len + 1) + 1 + 80
             + " PRIVMSG ".len() + target.len() + " :".len() + 2;
         let max_msg = 512_usize.saturating_sub(overhead);
 
@@ -249,24 +257,34 @@ impl State {
             return Err(io::Error::new(io::ErrorKind::InvalidInput, "target too long"));
         }
 
-        // Split on UTF-8 char boundaries
-        let mut remaining = msg;
-        while !remaining.is_empty() {
-            let split_at = if remaining.len() <= max_msg {
-                remaining.len()
-            } else {
-                // Find last char boundary at or before max_msg
-                let mut i = max_msg;
-                while i > 0 && !remaining.is_char_boundary(i) { i -= 1; }
-                // To avoid splitting mid-word, see if there was a space recently
-                let mut j = i;
-                while j > 1 && j > i-10 && remaining.as_bytes()[j] != b' ' { j -= 1; }
-                if remaining.as_bytes()[j] == b' ' { j }
-                else if i == 0 { max_msg } else { i }
-            };
-            let (chunk, rest) = remaining.split_at(split_at);
-            self.send_raw(&format!("PRIVMSG {target} :{chunk}")).await?;
-            remaining = rest;
+        for line in msg.split('\n') {
+            let mut remaining = line;
+            // Empty lines (blank paragraph breaks) can't be sent as empty
+            // PRIVMSGs - most IRC servers reject them. Skip.
+            if remaining.is_empty() { continue; }
+            loop {
+                let split_at = if remaining.len() <= max_msg {
+                    remaining.len()
+                } else {
+                    // Find last char boundary at or before max_msg.
+                    let mut i = max_msg;
+                    while i > 0 && !remaining.is_char_boundary(i) { i -= 1; }
+                    // Prefer splitting at a word boundary - look back up to
+                    // max_msg/4 chars for a space. With dense content (code)
+                    // we may not find one; fall back to the char boundary.
+                    let lookback = max_msg / 4;
+                    let bytes = remaining.as_bytes();
+                    let mut j = i;
+                    while j > 0 && (i - j) < lookback && bytes[j - 1] != b' ' {
+                        j -= 1;
+                    }
+                    if j > 0 && bytes[j - 1] == b' ' { j } else { i }
+                };
+                let (chunk, rest) = remaining.split_at(split_at);
+                self.send_raw(&format!("PRIVMSG {target} :{chunk}")).await?;
+                remaining = rest;
+                if remaining.is_empty() { break; }
+            }
         }
         Ok(())
     }