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/internal/byteslice/byteslice.go

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: Unlicense OR MIT
+
+// Package byteslice provides byte slice views of other Go values  such as
+// slices and structs.
+package byteslice
+
+import (
+	"reflect"
+	"unsafe"
+)
+
+// Struct returns a byte slice view of a struct.
+func Struct(s any) []byte {
+	v := reflect.ValueOf(s)
+	sz := int(v.Elem().Type().Size())
+	return unsafe.Slice((*byte)(unsafe.Pointer(v.Pointer())), sz)
+}
+
+// Uint32 returns a byte slice view of a uint32 slice.
+func Uint32(s []uint32) []byte {
+	n := len(s)
+	if n == 0 {
+		return nil
+	}
+	blen := n * int(unsafe.Sizeof(s[0]))
+	return unsafe.Slice((*byte)(unsafe.Pointer(&s[0])), blen)
+}
+
+// Slice returns a byte slice view of a slice.
+func Slice(s any) []byte {
+	v := reflect.ValueOf(s)
+	first := v.Index(0)
+	sz := int(first.Type().Size())
+	res := unsafe.Slice((*byte)(unsafe.Pointer(v.Pointer())), sz*v.Cap())
+	return res[:sz*v.Len()]
+}