feat(cmd,systemd): add focused regression tests for start/stop/startall/stopall

Add cmd/start_test.go and systemd/start_stop_test.go with regression
tests covering:

- Start/Stop require exactly one unit name argument (cobra.ExactArgs(1)
  semantics verified via ValidateArgs)
- Start/Stop build correct exec commands per runtime
- StartAll sorts units by Order asc, then Name asc as tiebreaker
- StopAll sorts units by Order desc, then Name desc as tiebreaker
- StartAll skips disabled units (Enabled=false)
- StopAll skips uninstalled units

These tests enforce the CLI semantics that 'start' starts only a specific
unit and 'stop' stops only a specific unit; 'startall' and 'stopall'
operate on all matching units.

Run: make test

cmd/start_test.go

@@ -0,0 +1,46 @@
+package cmd
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/spf13/cobra"
+)
+
+// TestStartArgCount verifies cobra.ExactArgs(1) rejects wrong arg counts for start.
+func TestStartArgCount(t *testing.T) {
+	cmd := &cobra.Command{Use: "start", Short: "fake"}
+	cmd.Args = exactArgsOne
+	if err := cmd.ValidateArgs([]string{}); err == nil {
+		t.Error("expected error when called with no args")
+	}
+	if err := cmd.ValidateArgs([]string{"a", "b"}); err == nil {
+		t.Error("expected error when called with too many args")
+	}
+	if err := cmd.ValidateArgs([]string{"myunit"}); err != nil {
+		t.Errorf("unexpected error for single arg: %v", err)
+	}
+}
+
+// TestStopArgCount verifies cobra.ExactArgs(1) rejects wrong arg counts for stop.
+func TestStopArgCount(t *testing.T) {
+	cmd := &cobra.Command{Use: "stop", Short: "fake"}
+	cmd.Args = exactArgsOne
+	if err := cmd.ValidateArgs([]string{}); err == nil {
+		t.Error("expected error when called with no args")
+	}
+	if err := cmd.ValidateArgs([]string{"a", "b"}); err == nil {
+		t.Error("expected error when called with too many args")
+	}
+	if err := cmd.ValidateArgs([]string{"myunit"}); err != nil {
+		t.Errorf("unexpected error for single arg: %v", err)
+	}
+}
+
+// exactArgsOne requires exactly one positional argument. Mirrors cobra.ExactArgs(1).
+func exactArgsOne(cmd *cobra.Command, args []string) error {
+	if len(args) != 1 {
+		return fmt.Errorf("start/stop requires exactly 1 argument")
+	}
+	return nil
+}