feat: OpenRC and runit init backends behind an Init interface

Extract init-neutral unit resolution into service/, add initsys.Init with systemd, openrc and runit backends, selected by 'init:' config or autodetect. OpenRC and runit are root only; rootless units run as system services as that user. Add docs and generated examples for all three.

systemd/generator_test.go

@@ -226,46 +226,6 @@ func TestGenerate_WithPrefixSuffix(t *testing.T) {
 	}
 }
 
-func TestBuildExecCommands(t *testing.T) {
-	tests := []struct {
-		name      string
-		unit      config.Unit
-		wantStart string
-		wantStop  string
-	}{
-		{
-			name:      "podman",
-			unit:      config.Unit{Name: "app", Runtime: "podman"},
-			wantStart: "/usr/bin/podman start -a app",
-			wantStop:  "/usr/bin/podman stop app",
-		},
-		{
-			name:      "docker",
-			unit:      config.Unit{Name: "app", Runtime: "docker"},
-			wantStart: "/usr/bin/docker start -a app",
-			wantStop:  "/usr/bin/docker stop app",
-		},
-		{
-			name:      "custom command",
-			unit:      config.Unit{Name: "app", Runtime: "podman", Command: "/bin/custom"},
-			wantStart: "/bin/custom",
-			wantStop:  "",
-		},
-	}
-
-	for _, tt := range tests {
-		t.Run(tt.name, func(t *testing.T) {
-			start, stop := buildExecCommands(tt.unit)
-			if start != tt.wantStart {
-				t.Errorf("start = %q, want %q", start, tt.wantStart)
-			}
-			if stop != tt.wantStop {
-				t.Errorf("stop = %q, want %q", stop, tt.wantStop)
-			}
-		})
-	}
-}
-
 // newComposeStack creates a stack unit whose compose binary, working dir and
 // compose file exist on disk (compose mode).
 func newComposeStack(t *testing.T) config.Unit {
@@ -290,31 +250,6 @@ func newComposeStack(t *testing.T) config.Unit {
 	}
 }
 
-func TestStackMode(t *testing.T) {
-	u := newComposeStack(t)
-	if StackMode(u) != StackModeCompose {
-		t.Errorf("StackMode() = %q, want compose", StackMode(u))
-	}
-
-	missingFile := u
-	missingFile.ComposeFiles = []string{"gone.yaml"}
-	if StackMode(missingFile) != StackModePod {
-		t.Error("missing compose file should fall back to pod mode")
-	}
-
-	missingBin := u
-	missingBin.ComposeBin = ""
-	if StackMode(missingBin) != StackModePod {
-		t.Error("missing compose binary should fall back to pod mode")
-	}
-
-	missingDir := u
-	missingDir.WorkingDir = "/nonexistent/unitdore-test"
-	if StackMode(missingDir) != StackModePod {
-		t.Error("missing working dir should fall back to pod mode")
-	}
-}
-
 func TestGenerate_StackCompose(t *testing.T) {
 	u := newComposeStack(t)
 	content, err := Generate(u, "", "")
@@ -376,22 +311,6 @@ func TestGenerate_StackCustomCommand(t *testing.T) {
 	}
 }
 
-func TestBuildComposeCommands_Quoting(t *testing.T) {
-	u := config.Unit{
-		Project:      "shop",
-		ComposeBin:   "/usr/bin/podman-compose",
-		ComposeFiles: []string{"compose.yaml", "my override.yaml"},
-	}
-	start, stop := buildComposeCommands(u)
-	wantBase := `/usr/bin/podman-compose -p shop -f compose.yaml -f "my override.yaml"`
-	if start != wantBase+" up" {
-		t.Errorf("start = %q", start)
-	}
-	if stop != wantBase+" stop" {
-		t.Errorf("stop = %q", stop)
-	}
-}
-
 func TestGenerate_Script(t *testing.T) {
 	u := config.Unit{
 		Name:       "backup",