build: emit xcframework for libghostty-vt on macOS

On Darwin targets, the build now automatically produces a universal
(arm64 + x86_64) XCFramework at lib/ghostty-vt.xcframework under
the install prefix. This bundles the fat static library with headers
so consumers using Xcode or Swift PM can link libghostty-vt directly.

build.zig

@@ -152,6 +152,18 @@ pub fn build(b: *std.Build) !void {
         ).step);
     }
 
+    // libghostty-vt xcframework (Apple only, universal binary)
+    if (config.target.result.os.tag.isDarwin()) {
+        const universal = try buildpkg.GhosttyLibVt.initStaticAppleUniversal(
+            b,
+            &config,
+            &deps,
+            &mod,
+        );
+        const xcframework = universal.xcframework();
+        b.getInstallStep().dependOn(xcframework.step);
+    }
+
     // Helpgen
     if (config.emit_helpgen) deps.help_strings.install();