build: respect config.emit_xcframework for building libghostty-vt.xcframework on Darwin

This fixes a hardcoded build issue on macOS where Zig unconditionally forces xcodebuild -create-xcframework to run during compilation, even when the caller explicitly specifies that they only want the raw standard C objects/headers (-Demit-lib-vt).

The Bug:
Around line 155 in build.zig, the libghostty-vt xcframework was being packaged unconditionally for Darwin builds. This caused developers (and wrappers like go-libghostty) attempting to natively build the vt library locally using only the minimal macOS Command Line Tools to experience an immediate crash, as xcodebuild -create-xcframework strictly demands a full Xcode application installation.

The Fix:
Guarded the GhosttyLibVt xcframework creation step with config.emit_xcframework. Because src/build/Config.zig intuitively forces emit_xcframework to default to false whenever emit_lib_vt is invoked, this structurally allows lightweight macOS builds to safely skip the xcodebuild invocation while still correctly compiling the standard .a object library files.

build.zig

@@ -155,7 +155,7 @@ pub fn build(b: *std.Build) !void {
     // libghostty-vt xcframework (Apple only, universal binary).
     // Only when building on macOS (not cross-compiling) since
     // xcodebuild is required.
-    if (builtin.os.tag.isDarwin() and config.target.result.os.tag.isDarwin()) {
+    if (config.emit_xcframework and builtin.os.tag.isDarwin() and config.target.result.os.tag.isDarwin()) {
         const apple_libs = try buildpkg.GhosttyLibVt.initStaticAppleUniversal(
             b,
             &config,