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/shader/gio/common.h

@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+struct m3x2 {
+	vec3 r0;
+	vec3 r1;
+};
+
+// fboTransform is the transformation that cancels the implied transformation
+// between the clip space and the framebuffer. Only two rows are returned. The
+// last is implied to be [0, 0, 1].
+const m3x2 fboTransform = m3x2(
+#if defined(LANG_HLSL) || defined(LANG_MSL) || defined(LANG_MSLIOS)
+	vec3(1.0, 0.0, 0.0),
+	vec3(0.0, -1.0, 0.0)
+#else
+	vec3(1.0, 0.0, 0.0),
+	vec3(0.0, 1.0, 0.0)
+#endif
+);
+
+// windowTransform is the transformation that cancels the implied transformation
+// between framebuffer space and window system coordinates.
+const m3x2 windowTransform = m3x2(
+#if defined(LANG_VULKAN)
+	vec3(1.0, 0.0, 0.0),
+	vec3(0.0, 1.0, 0.0)
+#else
+	vec3(1.0, 0.0, 0.0),
+	vec3(0.0, -1.0, 0.0)
+#endif
+);
+
+vec3 transform3x2(m3x2 t, vec3 v) {
+	return vec3(dot(t.r0, v), dot(t.r1, v), dot(vec3(0.0, 0.0, 1.0), v));
+}