feat(apprt): port the core/runtime contract types

Ports ZIG/src/apprt/{action,structs,surface,ipc,runtime}.zig into
ghostty-apprt: the Action enum performed by the core on the app runtime
(all 65 variants with docs), Target<S> generic over the runtime's
surface handle, clipboard/size/scale/selection structs, the core->surface
Message enum with a Mailbox over an AppMailbox trait, and IPC action
types for +new-window.

Owned Strings and Arc<Config> replace Zig's borrowed slices/pointers.
Scrollbar and RendererHealth are local stand-ins until core integration.
The embedded C API and browser runtime are out of scope.

Test Plan:
- cargo test -p ghostty-apprt (16 tests)

crates/ghostty-apprt/Cargo.toml

@@ -9,6 +9,7 @@ ghostty-vt.workspace = true
 ghostty-input.workspace = true
 ghostty-config.workspace = true
 ghostty-datastruct.workspace = true
+thiserror = "2.0.21"
 
 [lints]
 workspace = true