feat(vt): port Ghostty's VT parsing layer

Ports the parsing half of ZIG/src/terminal into the ghostty-vt crate:
- Parser state machine with a compile-time generated parse table and
  the UTF-8 decoder
- OSC parser incl. all sub-parsers (titles, colors, clipboard, hyperlinks,
  semantic prompts/OSC 133, OSC 9 / ConEmu, iTerm2, kitty clipboard,
  color and text-sizing protocols, context signals)
- CSI, SGR, modes table with save/restore, charsets, ansi, device status,
  mouse shapes, APC, DCS (incl. XTGETTCAP), tmux control-mode parser
- color palette / X11 color names, kitty color and graphics command parser

Deviations from Zig (documented in docs/rewrite/notes/ghostty-vt-W*.md):
parser actions own their payloads instead of borrowing parser buffers;
OSC strings are Vec<u8>; non-exhaustive Zig enums gain an Other(raw)
variant; tmux regexes replaced by equivalent hand-written matchers.

Known follow-up: semantic_prompt.rs carries local copies of the printf-q
and percent decoders that belong in ghostty-os.

Test Plan:
- cargo test -p ghostty-vt: 395 passed, 1 ignored (skipped in Zig too)
- cargo clippy -p ghostty-vt --all-targets -- -D warnings: clean

crates/ghostty-vt/src/osc/parsers/rxvt_extension.rs

@@ -0,0 +1,51 @@
+//! OSC 777 (ZIG/src/terminal/osc/parsers/rxvt_extension.zig).
+use crate::osc::{Command, Parser, State};
+
+/// Parse OSC 777
+pub fn parse(parser: &mut Parser, _terminator_ch: Option<u8>) -> Option<Command> {
+    let data = parser.data()?;
+    let Some(k) = data.iter().position(|&b| b == b';') else {
+        parser.state = State::Invalid;
+        return None;
+    };
+    let ext = &data[..k];
+    if ext != b"notify" {
+        log::warn!("unknown rxvt extension: {}", String::from_utf8_lossy(ext));
+        parser.state = State::Invalid;
+        return None;
+    }
+    let Some(t) = data[k + 1..]
+        .iter()
+        .position(|&b| b == b';')
+        .map(|i| i + k + 1)
+    else {
+        log::warn!("rxvt notify extension is missing the title");
+        parser.state = State::Invalid;
+        return None;
+    };
+    Some(Command::ShowDesktopNotification {
+        title: data[k + 1..t].to_vec(),
+        body: data[t + 1..].to_vec(),
+    })
+}
+
+#[cfg(test)]
+mod tests {
+    #![allow(clippy::unwrap_used)]
+    use super::*;
+
+    #[test]
+    fn osc_777_show_desktop_notification_with_title() {
+        let mut p = Parser::init(false);
+        for c in b"777;notify;Title;Body" {
+            p.next(*c);
+        }
+        assert_eq!(
+            p.end(Some(0x1b)).unwrap(),
+            Command::ShowDesktopNotification {
+                title: b"Title".to_vec(),
+                body: b"Body".to_vec()
+            }
+        );
+    }
+}