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/repeatable_string_map.rs

@@ -46,6 +46,14 @@ impl RepeatableStringMap {
             .map(|(_, v)| v.as_str())
     }
 
+    /// Insert or replace an entry (Zig: `map.put`).
+    pub fn put(&mut self, key: &str, value: &str) {
+        match self.map.iter_mut().find(|(k, _)| k == key) {
+            Some((_, v)) => value.clone_into(v),
+            None => self.map.push((key.to_owned(), value.to_owned())),
+        }
+    }
+
     /// Iterator over the entries in the map (insertion order).
     pub fn iter(&self) -> impl Iterator<Item = (&str, &str)> {
         self.map.iter().map(|(k, v)| (k.as_str(), v.as_str()))