refactor: extract base exporter, add sentinel errors, document unused fields

internal/exporters/base.go

@@ -0,0 +1,20 @@
+package exporters
+
+import "github.com/kjanat/articulate-parser/internal/interfaces"
+
+// baseExporter contains common fields shared by all exporter implementations.
+// Embed this struct in concrete exporters to inherit HTMLCleaner and Logger.
+type baseExporter struct {
+	// htmlCleaner is used to convert HTML content to plain text.
+	htmlCleaner interfaces.HTMLCleaner
+	// logger is used for logging warnings and errors.
+	logger interfaces.Logger
+}
+
+// newBaseExporter creates a new baseExporter with the given dependencies.
+func newBaseExporter(htmlCleaner interfaces.HTMLCleaner, logger interfaces.Logger) baseExporter {
+	return baseExporter{
+		htmlCleaner: htmlCleaner,
+		logger:      logger,
+	}
+}