feat(termio): port the IO thread, Exec backend and stream handler

Ports the remainder of ZIG/src/termio: Options, backend, message,
mailbox, Termio, Thread, Exec and stream_handler.

Threading model (replacing libxev): the IO thread owns Termio and runs a
single poll(2) loop over the mailbox eventfd, a stop eventfd, the child
pidfd and pty POLLOUT, driving Ghostty's timers (25 ms resize
coalescing, 1 s synchronized-output, 15 ms selection scroll, 200 ms
termios polling). A separate pty read thread locks the shared renderer
state and feeds the terminal. Renderer and surface interaction go
through RendererMailbox/SurfaceMailbox traits.

The stream handler answers all queries (DA, DSR, XTVERSION, DECRQM,
OSC color queries, XTGETTCAP, DECRQSS, ...) and forwards titles, pwd,
OSC 52, notifications, progress and bell as surface messages. Zig
unlocks the renderer mutex when a mailbox is full; here overflow goes to
an ordered outbox flushed after the locks drop.

IO1's local stand-ins are replaced with the real config types.

Open: RendererState lives in termio for now (renderer State.zig is
being ported); kitty graphics replies and tmux viewer are not hooked up;
wait-after-command lives in Surface (core); Flatpak stop path needs a
ghostty-os change.

Test Plan:
- cargo test -p ghostty-termio (102 unit + 21 end-to-end tests on a
  real pty, incl. backpressure, DA1 replies, resize coalescing, exits)

crates/ghostty-termio/src/wakeup.rs

@@ -0,0 +1,96 @@
+//! A tiny cross-thread wakeup primitive (replaces `xev.Async`).
+//!
+//! Backed by a non-blocking `eventfd`, so it can be included in a `poll(2)`
+//! set next to a pty fd or a pidfd. [`Wakeup::notify`] may be called from any
+//! thread, any number of times; the waiter calls [`Wakeup::drain`] after
+//! `poll` reports the fd readable.
+
+use std::io;
+use std::os::fd::{AsFd, BorrowedFd, OwnedFd};
+use std::sync::Arc;
+
+use rustix::event::{EventfdFlags, eventfd};
+
+/// See the module docs. Cloning shares the same underlying eventfd.
+#[derive(Clone, Debug)]
+pub struct Wakeup(Arc<OwnedFd>);
+
+impl Wakeup {
+    pub fn new() -> io::Result<Wakeup> {
+        let fd = eventfd(0, EventfdFlags::CLOEXEC | EventfdFlags::NONBLOCK)?;
+        Ok(Wakeup(Arc::new(fd)))
+    }
+
+    /// Wake up the waiter. Multiple notifications coalesce.
+    pub fn notify(&self) -> io::Result<()> {
+        loop {
+            match rustix::io::write(&*self.0, &1u64.to_ne_bytes()) {
+                Ok(_) => return Ok(()),
+                Err(rustix::io::Errno::INTR) => {}
+                // Counter would overflow: the waiter is already guaranteed
+                // to wake up.
+                Err(rustix::io::Errno::AGAIN) => return Ok(()),
+                Err(e) => return Err(e.into()),
+            }
+        }
+    }
+
+    /// Reset the notification state. Returns true if a notification was
+    /// pending.
+    pub fn drain(&self) -> bool {
+        let mut buf = [0u8; 8];
+        loop {
+            match rustix::io::read(&*self.0, &mut buf) {
+                Ok(_) => return true,
+                Err(rustix::io::Errno::INTR) => {}
+                Err(_) => return false,
+            }
+        }
+    }
+
+    /// The fd to poll for `POLLIN`.
+    pub fn as_fd(&self) -> BorrowedFd<'_> {
+        self.0.as_fd()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use rustix::event::{PollFd, PollFlags, poll};
+
+    fn readable(w: &Wakeup) -> bool {
+        let mut fds = [PollFd::from_borrowed_fd(w.as_fd(), PollFlags::IN)];
+        poll(
+            &mut fds,
+            Some(&rustix::time::Timespec {
+                tv_sec: 0,
+                tv_nsec: 0,
+            }),
+        )
+        .unwrap()
+            > 0
+    }
+
+    #[test]
+    fn notify_coalesces_and_drains() {
+        let w = Wakeup::new().unwrap();
+        assert!(!readable(&w));
+        w.notify().unwrap();
+        w.notify().unwrap();
+        assert!(readable(&w));
+        assert!(w.drain());
+        assert!(!readable(&w));
+        assert!(!w.drain());
+    }
+
+    #[test]
+    fn notify_from_other_thread() {
+        let w = Wakeup::new().unwrap();
+        let w2 = w.clone();
+        std::thread::spawn(move || w2.notify().unwrap())
+            .join()
+            .unwrap();
+        assert!(readable(&w));
+    }
+}