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)

Cargo.toml

@@ -50,6 +50,7 @@ ghostty-vt = { path = "crates/ghostty-vt" }
 ghostty-unicode = { path = "crates/ghostty-unicode" }
 ghostty-terminal = { path = "crates/ghostty-terminal" }
 ghostty-input = { path = "crates/ghostty-input" }
+ghostty-helpgen = { path = "crates/ghostty-helpgen" }
 ghostty-config = { path = "crates/ghostty-config" }
 ghostty-font = { path = "crates/ghostty-font" }
 ghostty-renderer = { path = "crates/ghostty-renderer" }
@@ -66,6 +67,7 @@ ghostty-bench = { path = "crates/ghostty-bench" }
 ghostty-inspector = { path = "crates/ghostty-inspector" }
 ghostty-vt-capi = { path = "crates/ghostty-vt-capi" }
 ghostty-wasm = { path = "crates/ghostty-wasm" }
+ghostty-lib = { path = "crates/ghostty-lib" }
 
 [workspace.lints.rust]
 unsafe_code = "forbid"