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/Cargo.toml

@@ -4,10 +4,52 @@ version.workspace = true
 edition.workspace = true
 license.workspace = true
 
+# NOTE: the `ghostty-config` and `ghostty-terminal` dependencies were removed
+# for now (config is being written concurrently, terminal is in progress).
+# TODO(integration): re-add them when Exec/Termio/stream_handler are ported and
+# replace the local stand-ins (shell_integration::{Command, ShellIntegrationFeatures},
+# command::{LinuxCgroup, GtkSingleInstance}).
 [dependencies]
-ghostty-config.workspace = true
-ghostty-terminal.workspace = true
 ghostty-os.workspace = true
+libc = "0.2.190"
+log = "0.4.34"
+nix = { version = "0.31.3", features = ["signal"] }
+rustix = { version = "1.1.5", features = ["fs", "process", "pty", "termios", "stdio"] }
+thiserror = "2.0.21"
 
-[lints]
-workspace = true
+# Termio forks/execs children and sets up ptys; unsafe is denied by default
+# and only allowed on minimal, SAFETY-commented items.
+[lints.rust]
+unsafe_code = "deny"
+
+[lints.clippy]
+pedantic = { level = "warn", priority = -1 }
+todo = "warn"
+unwrap_used = "warn"
+# Porting a low-level terminal emulator involves a lot of intentional
+# numeric casts and long state-machine functions; these pedantic lints are
+# noise for this codebase.
+cast_possible_truncation = "allow"
+cast_possible_wrap = "allow"
+cast_sign_loss = "allow"
+cast_precision_loss = "allow"
+cast_lossless = "allow"
+module_name_repetitions = "allow"
+too_many_lines = "allow"
+missing_errors_doc = "allow"
+missing_panics_doc = "allow"
+similar_names = "allow"
+struct_excessive_bools = "allow"
+fn_params_excessive_bools = "allow"
+match_same_arms = "allow"
+unreadable_literal = "allow"
+many_single_char_names = "allow"
+doc_markdown = "allow"
+must_use_candidate = "allow"
+return_self_not_must_use = "allow"
+used_underscore_binding = "allow"
+large_stack_arrays = "allow"
+inline_always = "allow"
+
+[dev-dependencies]
+temp-env = "0.3.6"