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-renderer/src/generic.rs

@@ -1520,6 +1520,12 @@ impl<G: GraphicsApi> Renderer<G> {
             .is_some_and(DisplayLink::is_running)
     }
 
+    /// The font grid currently used for rendering (for apprts that need
+    /// to read the primary face, e.g. macOS quicklook).
+    pub fn font_grid(&self) -> Arc<SharedGrid> {
+        Arc::clone(&lock(&self.draw).font_grid)
+    }
+
     /// Called by the renderer thread when it begins (`threadEnter`).
     pub fn thread_enter(&self) {
         lock(&self.draw).api.thread_enter();