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-config/src/c_value.rs

@@ -0,0 +1,64 @@
+//! The C representation of config values (Zig: `config/c_get.zig` together
+//! with the `cval` functions of the config types).
+//!
+//! `ghostty_config_get` writes a value of a key-dependent C type into a
+//! caller supplied pointer. This crate cannot use `unsafe`, so the typed
+//! value is described by [`CValue`] and the C API crate (`ghostty-lib`) does
+//! the actual write.
+
+use ghostty_input::command::CommandC;
+
+/// A size of the quick terminal along one axis in C form
+/// (`ghostty_quick_terminal_size_s`).
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum QuickTerminalSizeC {
+    None,
+    Percentage(f32),
+    Pixels(u32),
+}
+
+/// The value of a config field in the form the C API exposes it.
+#[derive(Debug, Clone, PartialEq)]
+pub enum CValue {
+    /// The type of this field is not supported by the C API (Zig `getValue`
+    /// returns false).
+    Unsupported,
+    /// An optional field without a value (returns false).
+    Null,
+    /// An optional string without a value: `ghostty_config_get` returns true
+    /// and stores a null pointer.
+    NullString,
+    /// A string (`?[:0]const u8`): stored as a NUL terminated `const char*`.
+    String(String),
+    /// A boolean (`bool`).
+    Bool(bool),
+    /// `u8` / `u32`, stored as `c_uint`.
+    Uint(u32),
+    /// `i16`, stored as `c_short`.
+    Short(i16),
+    /// `f32`.
+    F32(f32),
+    /// `f64`.
+    F64(f64),
+    /// `usize` (`Duration.cval`).
+    Usize(usize),
+    /// An enum: the tag name, stored as a NUL terminated `const char*`.
+    EnumName(&'static str),
+    /// A packed struct of at most 32 bits: its bits stored as `c_uint`.
+    Packed(u32),
+    /// `ghostty_config_color_s`.
+    Color { r: u8, g: u8, b: u8 },
+    /// `ghostty_config_color_list_s`.
+    ColorList(Vec<[u8; 3]>),
+    /// `ghostty_config_palette_s`.
+    Palette(Box<[[u8; 3]; 256]>),
+    /// `ghostty_config_command_list_s`.
+    Commands(Vec<CommandC>),
+    /// `ghostty_config_quick_terminal_size_s`.
+    QuickTerminalSize {
+        primary: QuickTerminalSizeC,
+        secondary: QuickTerminalSizeC,
+    },
+    /// `ghostty_config_path_s`.
+    Path { path: String, optional: bool },
+}