chore(v2): vendor dependencies for offline/China builds

go mod vendor pins onnxruntime_go v1.12.1, Gio and the rest into v2/vendor
so go run/build work without hitting proxy.golang.org (blocked/slow in
China). Verified: CGO_ENABLED=1 go build -mod=vendor ./internal/spike and
GOOS=windows go build -mod=vendor ./internal/ui both pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

v2/vendor/gioui.org/internal/cocoainit/cocoa_darwin.go

@@ -0,0 +1,21 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// Package cocoainit initializes support for multithreaded
+// programs in Cocoa.
+package cocoainit
+
+/*
+#cgo CFLAGS: -xobjective-c -fobjc-arc
+#cgo LDFLAGS: -framework Foundation
+#import <Foundation/Foundation.h>
+
+static inline void activate_cocoa_multithreading() {
+    [[NSThread new] start];
+}
+#pragma GCC visibility push(hidden)
+*/
+import "C"
+
+func init() {
+	C.activate_cocoa_multithreading()
+}