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/writer.go

@@ -20,6 +20,18 @@ type Writer interface {
 	WriteTable(table *models.Table) error
 }
 
+// NullableType constants control which Go package is used for nullable column types
+// in code-generation writers (Bun, GORM).
+const (
+	// NullableTypeResolveSpec uses github.com/bitechdev/ResolveSpec/pkg/spectypes
+	// (SqlString, SqlInt32, SqlVector, SqlStringArray, …). This is the default.
+	NullableTypeResolveSpec = "resolvespec"
+
+	// NullableTypeStdlib uses the standard library database/sql nullable types
+	// (sql.NullString, sql.NullInt32, …) and plain Go slices for arrays.
+	NullableTypeStdlib = "stdlib"
+)
+
 // WriterOptions contains common options for writers
 type WriterOptions struct {
 	// OutputPath is the path where the output should be written
@@ -33,6 +45,12 @@ type WriterOptions struct {
 	// Useful for databases like SQLite that do not support schemas.
 	FlattenSchema bool
 
+	// NullableTypes selects the Go type package used for nullable columns in
+	// code-generation writers (bun, gorm). Accepted values:
+	//   "resolvespec" (default) — github.com/bitechdev/ResolveSpec/pkg/spectypes
+	//   "stdlib"                — database/sql (sql.NullString, sql.NullInt32, …)
+	NullableTypes string
+
 	// Additional options can be added here as needed
 	Metadata map[string]interface{}
 }