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)

docs/rewrite/notes/ghostty-apprt-gtk.md

@@ -30,7 +30,7 @@ dependency (cycle): the core boundary is the trait set in `src/core.rs`.
 # Per-component notes (by sub-agent)
 ## F3: dialogs + config object (class/{dialog,config,config_errors_dialog,close_confirmation_dialog,clipboard_confirmation_dialog,title_dialog,debug_warning}.rs)
 
-Libadwaita >= 1.5 is assumed, so `Dialog` is always an `adw::AlertDialog` subclass (no `MessageDialog` fallback) and DebugWarning uses the `adw::Banner` (1.3) template; the clipboard dialog always uses the 1.4 template (remember switch). GType names, property names, signals and template children match the Zig classes.
+Update (GC): the Zig compat fallbacks are now ported, see compat-GC.md. By default (adw_1_5) `Dialog` is an `adw::AlertDialog` subclass, DebugWarning uses the Banner (1.3) template and the clipboard dialog the 1.4 template. GType names, property names, signals and template children match the Zig classes.
 
 Public API (all `&self` methods on `glib::wrapper!` types; signals via `connect_*`):
 - `config::Config` (GhosttyConfig; props `diagnostics-buffer`, `has-diagnostics` -> generated getters `diagnostics_buffer() -> Option<gtk::TextBuffer>`, `has_diagnostics() -> bool`): `new(&CoreConfig)` (clones), `from_arc(Arc<CoreConfig>)`, `new_default()`, `get() -> Arc<CoreConfig>`, `get_mut(|&mut CoreConfig| ..)` (Arc::make_mut), `clone_config() -> CoreConfig`. Test `ghostty_config` ported.