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)

Cargo.lock

@@ -321,6 +321,20 @@ name = "bytemuck"
 version = "1.25.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "95832e849adfb21180ccb6826a99da14e5d266ae5c2e668e1602cf234f153797"
+dependencies = [
+ "bytemuck_derive",
+]
+
+[[package]]
+name = "bytemuck_derive"
+version = "1.10.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f9abbd1bc6865053c427f7198e6af43bfdedc55ab791faed4fbd361d789575ff"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.119",
+]
 
 [[package]]
 name = "castaway"
@@ -876,6 +890,17 @@ dependencies = [
  "temp-dir",
 ]
 
+[[package]]
+name = "ghostty-apprt"
+version = "0.1.0"
+dependencies = [
+ "ghostty-config",
+ "ghostty-datastruct",
+ "ghostty-input",
+ "ghostty-vt",
+ "thiserror 2.0.21",
+]
+
 [[package]]
 name = "ghostty-apprt-gtk"
 version = "0.1.0"