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/zcover.frag.2.glsl150

@@ -0,0 +1,16 @@
+#version 150
+
+uniform sampler2D tex;
+uniform sampler2D cover;
+
+out vec4 fragColor;
+in vec2 vUV;
+in vec2 vCoverUV;
+
+void main()
+{
+    fragColor = texture(tex, vUV);
+    float c = min(abs(texture(cover, vCoverUV).x), 1.0);
+    fragColor *= c;
+}
+