Implement machine fingerprint hashing (T-501)

app-modern/platform/windows/machine_hash_stub.go

@@ -0,0 +1,8 @@
+//go:build !windows
+
+package windows
+
+// MachineHash is unavailable outside a Windows target.
+func MachineHash() (string, error) {
+	return "", ErrUnsupported
+}