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/op/paint/doc.go

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+/*
+Package paint provides drawing operations for 2D graphics.
+
+The PaintOp operation fills the current clip with the current brush, taking the
+current transformation into account. Drawing outside the current clip area is
+ignored.
+
+The current brush is set by either a ColorOp for a constant color, or
+ImageOp for an image, or LinearGradientOp for gradients.
+
+All color.NRGBA values are in the sRGB color space.
+*/
+package paint