feat(modules): add hello toy module as engine smoke test

internal/modules/hello/hello.go

@@ -0,0 +1,98 @@
+// Package hello is a toy module that writes a marker file. It exists to
+// validate the engine end-to-end before real modules arrive in Plan 3.
+package hello
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/MDietermann/ArduinoArchSetup/internal/module"
+)
+
+const defaultRelPath = ".cache/archduino/hello.ok"
+const markerContent = "archduino-hello\n"
+
+// Module is the exported module value. Path is overridable for tests.
+type Module struct {
+	Path string // absolute path; if empty, ~/.cache/archduino/hello.ok is used
+}
+
+func (m Module) resolve() (string, error) {
+	if m.Path != "" {
+		return m.Path, nil
+	}
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(home, defaultRelPath), nil
+}
+
+func (Module) Meta() module.Meta {
+	return module.Meta{
+		ID:          "hello",
+		Name:        "Hello",
+		Description: "Toy module that writes a marker file (engine smoke test).",
+	}
+}
+
+type state struct{ Present bool }
+
+func (m Module) Detect(ctx context.Context) (module.State, error) {
+	path, err := m.resolve()
+	if err != nil {
+		return nil, err
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return state{Present: false}, nil
+		}
+		return nil, err
+	}
+	return state{Present: string(data) == markerContent}, nil
+}
+
+func (m Module) Plan(ctx context.Context, cur module.State) (module.Plan, error) {
+	s, _ := cur.(state)
+	if s.Present {
+		return module.Plan{}, nil
+	}
+	path, err := m.resolve()
+	if err != nil {
+		return module.Plan{}, err
+	}
+	return module.Plan{Actions: []module.Action{{
+		Description: "write marker " + path,
+		Kind:        module.ActionFileWrite,
+		Exec: func(ctx context.Context) error {
+			if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+				return err
+			}
+			return os.WriteFile(path, []byte(markerContent), 0o644)
+		},
+	}}}, nil
+}
+
+func (m Module) Apply(ctx context.Context, p module.Plan, r module.Reporter) error {
+	for i, a := range p.Actions {
+		r.Progress(m.Meta().ID, i+1, len(p.Actions), a.Description)
+		if err := a.Exec(ctx); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (m Module) Verify(ctx context.Context) error {
+	st, err := m.Detect(ctx)
+	if err != nil {
+		return err
+	}
+	if !st.(state).Present {
+		return fmt.Errorf("hello: marker not present after apply")
+	}
+	return nil
+}