Centralize deterministic JSON file writing

internal/jsonfile/jsonfile.go

@@ -0,0 +1,28 @@
+package jsonfile
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+)
+
+// Write creates or truncates path and writes deterministic indented JSON.
+func Write(path string, value any) (err error) {
+	file, err := os.Create(path)
+	if err != nil {
+		return fmt.Errorf("create %q: %w", path, err)
+	}
+	defer func() {
+		closeErr := file.Close()
+		if err == nil && closeErr != nil {
+			err = fmt.Errorf("close %q: %w", path, closeErr)
+		}
+	}()
+
+	encoder := json.NewEncoder(file)
+	encoder.SetIndent("", "  ")
+	if err := encoder.Encode(value); err != nil {
+		return fmt.Errorf("encode %q: %w", path, err)
+	}
+	return nil
+}