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/readers/pgsql/queries.go

@@ -252,7 +252,7 @@ func (r *Reader) queryColumns(schemaName string) (map[string]map[string]*models.
 				column.AutoIncrement = true
 				column.Default = defaultVal
 			} else {
-				column.Default = defaultVal
+				column.Default = normalizePostgresDefault(defaultVal)
 			}
 		}
 
@@ -613,3 +613,30 @@ func (r *Reader) parseIndexDefinition(indexName, tableName, schema, indexDef str
 
 	return index, nil
 }
+
+// normalizePostgresDefault converts a raw PostgreSQL column_default expression into the
+// unquoted string value that the model convention expects. PostgreSQL stores string
+// literal defaults as 'value' or 'value'::type (e.g. '{}'::text[]), while every other
+// reader stores the bare value so the writer can re-quote it correctly.
+func normalizePostgresDefault(defaultVal string) string {
+	if !strings.HasPrefix(defaultVal, "'") {
+		return defaultVal
+	}
+	// Decode the SQL string literal: skip the leading quote, unescape '' → ', stop at
+	// the first unescaped closing quote (any trailing ::cast is ignored).
+	rest := defaultVal[1:]
+	var buf strings.Builder
+	for i := 0; i < len(rest); i++ {
+		if rest[i] == '\'' {
+			if i+1 < len(rest) && rest[i+1] == '\'' {
+				buf.WriteByte('\'')
+				i++
+			} else {
+				break
+			}
+		} else {
+			buf.WriteByte(rest[i])
+		}
+	}
+	return buf.String()
+}