feat(lib): port libghostty, Ghostty's embedded C API

Ports apprt/embedded.zig, main_c.zig and config/CApi.zig + c_get.zig as
crates/ghostty-lib (cdylib/staticlib/rlib `libghostty`), exporting every
function of the unchanged include/ghostty.h - the ABI Ghostty's Swift
macOS/iOS app links against. The embedded runtime implements the
runtime traits over the host's C callbacks, so the real core drives it.

- ghostty-core selects the renderer per target: Metal on Apple (surface
  NSView/UIView via RtSurface::platform_view), OpenGL elsewhere.
- ghostty_config_get follows Zig's c_get rules via a CValue type (143
  supported keys, checked against the Zig field types).
- The inspector C API is complete; its Metal backend replaces ImGui's
  Metal renderer and has not run.

Verification: generated C ABI probe (sizeof/alignof/offsetof of every
struct/union, 755 enum values, every function linked, nm exports, headers
byte-identical to upstream); a C host on Linux covering init, CLI
actions, config, app, keys, focus and color scheme; an opt-in
test-platform feature creating real surfaces on Linux with headless EGL
and a pty (13 tests). Apple targets are clippy-clean but not run; the
Swift app has not been built against it.

Test Plan:
- cargo test -p ghostty-lib (and --features test-platform)
- cargo clippy --target aarch64-apple-darwin -p ghostty-lib (cross env
  in docs/rewrite/notes/macos-M1.md)

crates/ghostty-lib/tests/c_host.rs

@@ -0,0 +1,81 @@
+//! Functional test: a C program using the unchanged `ghostty.h` (init,
+//! CLI actions, config get/load/finalize/diagnostics, app lifecycle,
+//! key bindings, actions callback) compiled with `cc` and linked against
+//! the built `libghostty`.
+#![cfg(unix)]
+#![allow(clippy::unwrap_used)]
+
+mod common;
+
+use std::{path::PathBuf, process::Command};
+
+fn host() -> PathBuf {
+    static ONCE: std::sync::OnceLock<PathBuf> = std::sync::OnceLock::new();
+    ONCE.get_or_init(|| {
+        let lib_dir = common::build_lib();
+        let dir = common::scratch_dir("host");
+        let exe = dir.join("host");
+        let src = common::manifest_dir().join("tests/c/host.c");
+        common::compile_c(&lib_dir, &[&src], &exe, &["-Wall", "-Wextra", "-pthread"]);
+        exe
+    })
+    .clone()
+}
+
+fn run(args: &[&str]) -> std::process::Output {
+    let dir = tempfile::tempdir().unwrap();
+    Command::new(host())
+        .args(args)
+        .env("XDG_CONFIG_HOME", dir.path())
+        .env("HOME", dir.path())
+        .env_remove("GHOSTTY_LOG")
+        .output()
+        .unwrap()
+}
+
+#[test]
+fn host_program_passes() {
+    let out = run(&[]);
+    let stdout = String::from_utf8_lossy(&out.stdout);
+    let stderr = String::from_utf8_lossy(&out.stderr);
+    assert!(
+        out.status.success(),
+        "host failed\nstdout:\n{stdout}\nstderr:\n{stderr}"
+    );
+    assert!(stdout.contains("OK"), "stdout:\n{stdout}");
+    assert!(stdout.contains("version=1.3.1"), "stdout:\n{stdout}");
+}
+
+#[test]
+fn cli_try_action_runs_version_and_exits() {
+    let out = run(&["+version"]);
+    let stdout = String::from_utf8_lossy(&out.stdout);
+    assert!(
+        out.status.success(),
+        "stderr: {}",
+        String::from_utf8_lossy(&out.stderr)
+    );
+    assert!(stdout.starts_with("Ghostty 1.3.1"), "stdout:\n{stdout}");
+    // The action ran instead of the rest of the program.
+    assert!(!stdout.contains("OK"));
+}
+
+#[test]
+fn cli_try_action_runs_other_actions() {
+    let out = run(&["+list-keybinds"]);
+    assert!(out.status.success());
+    let stdout = String::from_utf8_lossy(&out.stdout);
+    assert!(stdout.contains("new_window"), "stdout:\n{stdout}");
+
+    let out = run(&["+list-actions"]);
+    assert!(out.status.success());
+    assert!(String::from_utf8_lossy(&out.stdout).contains("new_window"));
+}
+
+#[test]
+fn init_rejects_multiple_actions() {
+    let out = run(&["+version", "+help"]);
+    // ghostty_init fails with a non-zero code, which the host CHECKs; the
+    // program then still runs on.
+    assert!(!out.status.success());
+}