feat: full NFSv4.2 server and client in pure Go

Assisted-by: GLM 5.3 Flash

internal/nfsfs/mknod_linux.go

@@ -0,0 +1,25 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+package nfsfs
+
+import "syscall"
+
+// mknod creates a device node. It needs CAP_MKNOD on Linux, and a failure
+// of permission is reported as such rather than as an io error. The kind
+// is picked before the single call: a second mknod over an existing node
+// fails EEXIST and leaves the wrong kind behind.
+func mknod(path string, spec CreateSpec, perm uint32) error {
+	dev := int(makedev(spec.Major, spec.Minor))
+	kind := uint32(syscall.S_IFCHR)
+	if spec.Kind == KindBlk {
+		kind = syscall.S_IFBLK
+	}
+	return syscall.Mknod(path, perm|kind, dev)
+}
+
+// makedev assembles a device number the way the kernel expects it.
+func makedev(major, minor uint32) uint64 {
+	return uint64(minor&0xff) | uint64(major&0xfff)<<8 |
+		uint64(minor&0xfff00)<<12 | uint64(major&0xfffff000)<<32
+}