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

Assisted-by: GLM 5.3 Flash

internal/nfsfs/sparse_stub.go

@@ -0,0 +1,22 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+//go:build !linux && !freebsd
+
+package nfsfs
+
+// SeekHole needs a system hole seeking call.
+func (l *Local) SeekHole(h Handle, offset int64) (int64, bool, error) {
+	return 0, false, ErrNoSparse
+}
+
+// SeekData needs a system hole seeking call.
+func (l *Local) SeekData(h Handle, offset int64) (int64, bool, error) {
+	return 0, false, ErrNoSparse
+}
+
+// Allocate needs a system space reservation call.
+func (l *Local) Allocate(h Handle, offset, length int64) error { return ErrNoSparse }
+
+// Deallocate needs a system hole punching call.
+func (l *Local) Deallocate(h Handle, offset, length int64) error { return ErrNoSparse }