feat(termio): port pty, child command spawning, shell integration

Ports the parts of Ghostty's IO layer that don't depend on the terminal:
- pty.zig: pty pair, window size, mode, child-side setsid/TIOCSCTTY;
  master is CLOEXEC with IUTF8 set
- Command.zig: fork/execvpe with Ghostty's exact child setup (stdio
  dup2, best-effort chdir, rlimit restore, os/runtime pre-exec hooks,
  cgroup handling) and exit status decoding
- shell_integration.zig for bash/zsh/fish/elvish/nushell plus feature
  flags; scripts copied verbatim to resources/shell-integration/
- terminfo generator for xterm-ghostty, byte-identical to Zig's encode
  output (golden file produced with the real Zig code)

This crate opts into unsafe_code = "deny" (fork/exec need it) with
allow + SAFETY comments on minimal items.

Integration left: config-derived option types are local stand-ins;
cgroup D-Bus scope creation is a closure hook; Exec/Termio/Thread/
stream_handler wait for ghostty-terminal. Pty now closes both fds on
drop (Zig never auto-closed the slave) - to re-check when wiring Exec.

Test Plan:
- cargo test -p ghostty-termio (52 tests, incl. real pty spawn test
  expecting "hi\r\n31 97\r\n" from `echo hi; stty size`)

crates/ghostty-termio/tests/terminfo_golden.rs

@@ -0,0 +1,34 @@
+//! The terminfo source emitted by the Rust port must be byte-identical to the
+
+#![allow(clippy::unwrap_used)]
+//! output of the Zig `terminfo/ghostty.zig` `encode` (golden file generated
+//! with the Zig reference implementation).
+
+use ghostty_termio::terminfo;
+
+#[test]
+fn terminfo_source_matches_zig_output() {
+    let expected = include_str!("data/xterm-ghostty.terminfo");
+    assert_eq!(expected, terminfo::ghostty.encode_to_string());
+}
+
+#[test]
+fn terminfo_compiles_with_tic_if_available() {
+    let Ok(dir) = std::env::var("CARGO_TARGET_TMPDIR") else {
+        return;
+    };
+    let src = std::path::Path::new(&dir).join("xterm-ghostty.src");
+    std::fs::write(&src, terminfo::ghostty.encode_to_string()).unwrap();
+    let out = std::path::Path::new(&dir).join("terminfo-out");
+    let _ = std::fs::create_dir_all(&out);
+    let Ok(status) = std::process::Command::new("tic")
+        .arg("-x")
+        .arg("-o")
+        .arg(&out)
+        .arg(&src)
+        .status()
+    else {
+        return; // tic not installed
+    };
+    assert!(status.success());
+}