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-font/src/face/coretext.rs

@@ -697,6 +697,20 @@ impl Face {
         Ok(util::font_table_bytes(&self.font, tag))
     }
 
+    /// A copy of this face's font at `size / scale` points: the font to use
+    /// for macOS quicklook (`ghostty_surface_quicklook_font`). The content
+    /// scale is applied to our font stack because we render at 1x, but the
+    /// callers of the quicklook font apply the scale themselves.
+    pub fn quicklook_font(&self, scale: f64) -> CFRetained<CTFont> {
+        // SAFETY: plain CoreText getter; null matrix and no descriptor are
+        // allowed for the copy.
+        unsafe {
+            let size = self.font.size();
+            self.font
+                .copy_with_attributes(size / scale, ptr::null(), None)
+        }
+    }
+
     /// The underlying CTFont (for shapers).
     pub(crate) fn ct_font(&self) -> &CTFont {
         &self.font