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/widget/doc.go

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// Package widget implements state tracking and event handling of
+// common user interface controls. To draw widgets, use a theme
+// packages such as package [gioui.org/widget/material].
+package widget