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.

src/build/GhosttyZig.zig

@@ -24,6 +24,44 @@ pub fn init(
     b: *std.Build,
     cfg: *const Config,
     deps: *const SharedDeps,
+) !GhosttyZig {
+    return initInner(b, cfg, deps, "ghostty-vt", "ghostty-vt-c");
+}
+
+/// Create a new GhosttyZig with modules retargeted to a different
+/// architecture. Used to produce universal (fat) binaries on macOS.
+pub fn retarget(
+    self: *const GhosttyZig,
+    b: *std.Build,
+    cfg: *const Config,
+    deps: *const SharedDeps,
+    target: std.Build.ResolvedTarget,
+) !GhosttyZig {
+    _ = self;
+    const retargeted_config = try b.allocator.create(Config);
+    retargeted_config.* = cfg.*;
+    retargeted_config.target = target;
+
+    const retargeted_deps = try b.allocator.create(SharedDeps);
+    retargeted_deps.* = try deps.retarget(b, target);
+
+    // Use unique module names to avoid collisions with the original target.
+    const arch_name = @tagName(target.result.cpu.arch);
+    return initInner(
+        b,
+        retargeted_config,
+        retargeted_deps,
+        b.fmt("ghostty-vt-{s}", .{arch_name}),
+        b.fmt("ghostty-vt-c-{s}", .{arch_name}),
+    );
+}
+
+fn initInner(
+    b: *std.Build,
+    cfg: *const Config,
+    deps: *const SharedDeps,
+    vt_name: []const u8,
+    vt_c_name: []const u8,
 ) !GhosttyZig {
     // Terminal module build options
     var vt_options = cfg.terminalOptions(.lib);
@@ -37,7 +75,7 @@ pub fn init(
 
     return .{
         .vt = try initVt(
-            "ghostty-vt",
+            vt_name,
             b,
             cfg,
             deps,
@@ -46,7 +84,7 @@ pub fn init(
         ),
 
         .vt_c = try initVt(
-            "ghostty-vt-c",
+            vt_c_name,
             b,
             cfg,
             deps,