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/README.md

@@ -0,0 +1,18 @@
+# GPU programs for the Gio project
+
+This repository contains source code for the [Gio](https://gioui.org)
+project. It also contains the generators and derived versions for use with the
+GPU APIs supported by Gio.
+
+# Generating CPU fallbacks
+
+The `piet/gencpu.sh` script updates the piet-gpu binaries:
+
+```
+$ cd piet
+$ ./gencpu.sh
+```
+
+## Issues and contributions
+
+See the [Gio contribution guide](https://gioui.org/doc/contribute).