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/path.rs

@@ -6,6 +6,7 @@ use std::{
 };
 
 use crate::{
+    c_value::CValue,
     cli_args::{ArgError, ConfigField},
     diagnostics::{Diagnostic, DiagnosticList},
     formatter::EntryFormatter,
@@ -171,6 +172,19 @@ impl Path {
 }
 
 impl ConfigField for Path {
+    fn c_value(&self) -> CValue {
+        match self {
+            Path::Optional(p) => CValue::Path {
+                path: p.clone(),
+                optional: true,
+            },
+            Path::Required(p) => CValue::Path {
+                path: p.clone(),
+                optional: false,
+            },
+        }
+    }
+
     /// Parse CLI option. The value will be overwritten.
     fn parse_cli(&mut self, input: Option<&str>) -> Result<(), ArgError> {
         let Some(item) = Path::parse(input)? else {