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/windows-WIN.md

@@ -95,9 +95,9 @@ Unix-specific existing tests are `cfg(unix)`-gated.
 * `Command::wait(block)` ignores `block` (blocks, like Zig). The thread
   handle from `CreateProcessW` is closed (Zig leaks it).
 * Fontconfig discovery is kept on Windows (needs a real fontconfig to link).
-* Not touched: `cli/ssh-cache/DiskCache` file locking, `cli/boo|list_colors|
-  list_keybinds|edit_config` Windows branches (ghostty-cli), `benchmark/
-  CodepointWidth`, `config/Config.zig` `probableCliEnvironment` env-based
+* (Update GC: `cli/ssh-cache/DiskCache` locking, the `cli/boo|list_colors|
+  list_keybinds|edit_config` Windows branches and `benchmark/CodepointWidth` are
+  now done, see compat-GC*.md.) Not touched: `config/Config.zig` `probableCliEnvironment` env-based
   extras. ghostty-core/apprt-gtk are not Windows-checkable (GTK).
 * Unrelated breakage seen while verifying (other agents): `ghostty-config`
   `config.rs` uses `ghostty_os::launched_from_desktop` under `cfg!(macos)`