feat: 新增 disk usage 内置命令（跨平台 du 等价）

- 注册表新增 Variant.Builtin 内置动作，与 Argv 互斥并参与平台匹配
- disk.usage 支持位置参数与 --path/--path=，输出按大小降序并带合计
- Unix 用设备号实现 -x 不跨文件系统，Windows 降级为不跟随 reparse point
- list/help/explain 支持展示内置动作，补充单元与 CLI 测试、README

internal/executor/device_windows.go

@@ -0,0 +1,12 @@
+//go:build windows
+
+package executor
+
+import "os"
+
+// Windows has no stable per-file device ID in os.FileInfo. Directory walks do
+// not cross volumes on their own, so -x degrades to "do not follow reparse
+// points", which filepath.WalkDir already guarantees.
+func deviceOf(os.FileInfo) (uint64, bool) { return 0, false }
+
+func deviceOfPath(string) (uint64, bool) { return 0, false }