feat: usage example

Assisted-by: GLM 5.3 Flash

examples/basic/main_test.go

@@ -0,0 +1,56 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: MIT
+
+package main
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestRunPrintsConfigAndMarshal(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	if code := Run(&stdout, &stderr); code != 0 {
+		t.Fatalf("Run returned %d, stderr = %q", code, stderr.String())
+	}
+	got := stdout.String()
+	for _, want := range []string{
+		"title:    interpres demo",
+		"launched: 2024-11-04T09:00:00Z",
+		"debug:    false",
+		"server:   127.0.0.1:9090",
+		"user:     petr",
+		"admin=true",
+		"admin=false",
+		"--- marshal (group by kind, default) ---",
+		`title = "interpres demo"`,
+		"--- marshal (GroupByKind=false) ---",
+		"[server]",
+		"port = 9090",
+		"[[users]]",
+		"--- round-trip ---",
+	} {
+		if !strings.Contains(got, want) {
+			t.Errorf("output missing %q\n----\n%s", want, got)
+		}
+	}
+}
+
+func TestRunMarshalerFieldRendersAsScalar(t *testing.T) {
+	// The Port Marshaler hook returns a string, so the field becomes a
+	// scalar "9000:9001" inside [srv] rather than an integer.
+	var stdout, stderr bytes.Buffer
+	if code := Run(&stdout, &stderr); code != 0 {
+		t.Fatalf("Run returned %d, stderr = %q", code, stderr.String())
+	}
+	for _, want := range []string{
+		`port = "9000:9001"`,
+		`addr = "10.0.0.1:8080"`,
+		`tag = "primary"`,
+	} {
+		if !strings.Contains(stdout.String(), want) {
+			t.Errorf("output missing %q\n----\n%s", want, stdout.String())
+		}
+	}
+}