feat: add OCR document types and ports

OcrDocumentElement extends the layout element with text; OcrDocumentResult stays independent because the parent's defaulted call_stats forbids a required markdown field. OcrDocumentTransport is a separate Protocol so existing OcrTransport implementers keep passing isinstance.

tests/unit/test_package.py

@@ -22,6 +22,8 @@ def test_ocr_public_surface_exported():
         "OcrTextResult",
         "OcrLayoutResult",
         "OcrLayoutElement",
+        "OcrDocumentResult",
+        "OcrDocumentElement",
     ):
         assert hasattr(polygateway, name), name
         assert name in polygateway.__all__, name