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`)

resources/shell-integration/README.md

@@ -0,0 +1,117 @@
+# Shell Integration Code
+
+This is the shell-specific shell-integration code that is
+used for the shell-integration feature set that Ghostty
+supports.
+
+This README is meant as developer documentation and not as
+user documentation. For user documentation, see the main
+README or [ghostty.org](https://ghostty.org/docs)
+
+## Implementation Details
+
+### Bash
+
+Automatic [Bash](https://www.gnu.org/software/bash/) shell integration works by
+starting Bash in POSIX mode and using the `ENV` environment variable to load
+our integration script (`bash/ghostty.bash`). This prevents Bash from loading
+its normal startup files, which becomes our script's responsibility (along with
+disabling POSIX mode).
+
+Bash shell integration can also be sourced manually from `bash/ghostty.bash`.
+This also works for older versions of Bash.
+
+```bash
+# Ghostty shell integration for Bash. This must be at the top of your bashrc!
+if [ -n "${GHOSTTY_RESOURCES_DIR}" ]; then
+    builtin source "${GHOSTTY_RESOURCES_DIR}/shell-integration/bash/ghostty.bash"
+fi
+```
+
+> [!NOTE]
+>
+> The version of Bash distributed with macOS (`/bin/bash`) does not support
+> automatic shell integration. You'll need to manually source the shell
+> integration script (as shown above). You can also install a standard
+> version of Bash from Homebrew or elsewhere and set it as your shell.
+
+### Elvish
+
+For [Elvish](https://elv.sh), `$GHOSTTY_RESOURCES_DIR/src/shell-integration`
+contains an `./elvish/lib/ghostty-integration.elv` file.
+
+Elvish, on startup, searches for paths defined in `XDG_DATA_DIRS`
+variable for `./elvish/lib/*.elv` files and imports them. They are thus
+made available for use as modules by way of `use <filename>`.
+
+Ghostty launches Elvish, passing the environment with `XDG_DATA_DIRS` prepended
+with `$GHOSTTY_RESOURCES_DIR/src/shell-integration`. It contains
+`./elvish/lib/ghostty-integration.elv`. The user can then import it
+by `use ghostty-integration` every time after shell startup or
+autostart integration in `$XDG_CONFIG_HOME/elvish/rc.elv`,
+which will run the integration routines.
+
+If you decide to autostart `ghostty-integration` with `rc.elv`, you should
+detect whether the terminal is Ghostty or not. To do this, add this to the end
+of your `rc.elv` file:
+
+```elvish
+if (eq $E:TERM "xterm-ghostty") {
+  try { use ghostty-integration } catch { }
+}
+```
+
+The [Elvish](https://elv.sh) shell integration is supported by
+the community and is not officially supported by Ghostty. We distribute
+it for ease of access and use but do not provide support for it.
+If you experience issues with the Elvish shell integration, I welcome
+any contributions to fix them. Thank you!
+
+### Fish
+
+For [Fish](https://fishshell.com/), Ghostty prepends to the
+`XDG_DATA_DIRS` directory. Fish automatically loads configuration
+files in `<XDG_DATA_DIR>/fish/vendor_conf.d/*.fish` on startup,
+allowing us to automatically integrate with the shell. For details
+on the Fish startup process, see the
+[Fish documentation](https://fishshell.com/docs/current/language.html).
+
+### Nushell
+
+For [Nushell](https://www.nushell.sh/), Ghostty prepends to the
+`XDG_DATA_DIRS` directory, making the `ghostty` module available through
+Nushell's vendor autoload mechanism. Ghostty then automatically imports
+the module using the `-e "use ghostty *"` flag when starting Nushell.
+
+Nushell provides many shell features itself, such as `title` and `cursor`,
+so our integration focuses on Ghostty-specific features like `sudo`,
+`ssh-env`, and `ssh-terminfo`.
+
+The shell integration is automatically enabled when running Nushell in Ghostty,
+but you can also load it manually is shell integration is disabled:
+
+```nushell
+source $GHOSTTY_RESOURCES_DIR/shell-integration/nushell/vendor/autoload/ghostty.nu
+use ghostty *
+```
+
+### Zsh
+
+Automatic [Zsh](https://www.zsh.org/) integration works by temporarily setting
+`ZDOTDIR` to our `zsh` directory. An existing `ZDOTDIR` environment variable
+value will be retained and restored after our shell integration scripts are
+run.
+
+However, if `ZDOTDIR` is set in a system-wide file like `/etc/zshenv`, it will
+override Ghostty's `ZDOTDIR` value, preventing the shell integration from being
+loaded. In this case, the shell integration needs to be loaded manually.
+
+To load the Zsh shell integration manually:
+
+```zsh
+if [[ -n $GHOSTTY_RESOURCES_DIR ]]; then
+  source "$GHOSTTY_RESOURCES_DIR"/shell-integration/zsh/ghostty-integration
+fi
+```
+
+Shell integration requires Zsh 5.1+.