feat: add --types flag and stdlib nullable type support for bun/gorm writers

* Fix pgsql reader double-quoting defaults: normalizePostgresDefault strips
  surrounding SQL string literal quotes from column_default before storing,
  matching the convention used by every other reader.

* Add NullableTypes field to WriterOptions with NullableTypeResolveSpec
  (default) and NullableTypeStdlib constants.

* Both bun and gorm TypeMappers now accept a typeStyle parameter. stdlib
  mode produces sql.NullString/NullInt32/NullTime etc. for nullable scalars,
  plain Go slices for arrays, and time.Time for NOT NULL timestamps. Default
  resolvespec behaviour is unchanged.

* Add --types flag to convert and split commands.

* Update bun/README.md and gorm/README.md with side-by-side generated code
  examples, updated type mapping tables, and Writer Options documentation.

pkg/writers/bun/writer_test.go

@@ -556,7 +556,7 @@ func TestWriter_FieldNameCollision(t *testing.T) {
 }
 
 func TestTypeMapper_SQLTypeToGoType_Bun(t *testing.T) {
-	mapper := NewTypeMapper()
+	mapper := NewTypeMapper("")
 
 	tests := []struct {
 		sqlType string
@@ -587,7 +587,7 @@ func TestTypeMapper_SQLTypeToGoType_Bun(t *testing.T) {
 }
 
 func TestTypeMapper_BuildBunTag(t *testing.T) {
-	mapper := NewTypeMapper()
+	mapper := NewTypeMapper("")
 
 	tests := []struct {
 		name   string
@@ -700,7 +700,7 @@ func TestTypeMapper_BuildBunTag(t *testing.T) {
 }
 
 func TestTypeMapper_BuildBunTag_PreservesExplicitTypeModifiers(t *testing.T) {
-	mapper := NewTypeMapper()
+	mapper := NewTypeMapper("")
 
 	col := &models.Column{
 		Name:      "embedding",