Add stage planning and placeholder runner

internal/app/planner.go

@@ -0,0 +1,21 @@
+package app
+
+import (
+	"fmt"
+
+	"gitea.maximumdirect.net/eric/narratio/internal/stage"
+)
+
+// BuildFullPlan returns the canonical full stage list in deterministic order.
+func BuildFullPlan() []stage.Stage {
+	return stage.All()
+}
+
+// BuildSingleStagePlan returns a one-stage plan for an exact stage name.
+func BuildSingleStagePlan(name string) ([]stage.Stage, error) {
+	s, err := stage.Select(name)
+	if err != nil {
+		return nil, fmt.Errorf("build stage plan: %w", err)
+	}
+	return []stage.Stage{s}, nil
+}