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/cover.frag

@@ -0,0 +1,20 @@
+#version 310 es
+
+// SPDX-License-Identifier: Unlicense OR MIT
+
+precision mediump float;
+
+{{.Header}}
+
+layout(location = 0) in highp vec2 vCoverUV;
+layout(location = 1) in highp vec2 vUV;
+
+layout(binding = 1) uniform sampler2D cover;
+
+layout(location = 0) out vec4 fragColor;
+
+void main() {
+    fragColor = {{.FetchColorExpr}};
+	float c = min(abs(texture(cover, vCoverUV).r), 1.0);
+	fragColor *= c;
+}