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_ports.py

@@ -12,6 +12,7 @@ from polygateway.ports import (
     GateState,
     GateUpdate,
     Middleware,
+    OcrDocumentTransport,
     OcrTransport,
     Permit,
     ProviderGate,
@@ -193,6 +194,7 @@ class TestReasoningTierIsOnlyOnTheChatPort:
             (EmbeddingTransport, "embed"),
             (OcrTransport, "recognize_text"),
             (OcrTransport, "parse_layout"),
+            (OcrDocumentTransport, "parse_document"),
         ],
     )
     def test_other_transports_have_no_reasoning_tier(self, protocol, method):
@@ -372,6 +374,34 @@ class TestOcrPorts:
         assert isinstance(GoodClient(), OcrLayoutPort)
         assert isinstance(GoodTransport(), OcrTransport)
 
+    def test_document_ports_are_runtime_checkable(self):
+        from polygateway.ports import OcrDocumentPort, OcrDocumentTransport
+
+        class GoodClient:
+            async def parse_document(self, document): ...
+
+        class GoodTransport:
+            async def parse_document(self, *, document, source, call_id): ...
+
+        class Empty: ...
+
+        assert isinstance(GoodClient(), OcrDocumentPort)
+        assert isinstance(GoodTransport(), OcrDocumentTransport)
+        assert not isinstance(Empty(), OcrDocumentPort)
+        assert not isinstance(Empty(), OcrDocumentTransport)
+
+    def test_ocr_transport_members_are_unchanged(self):
+        """新能力走独立 Protocol: 往旧 runtime_checkable Protocol 加成员,会让所有
+        现有实现(测试替身、下游自带 fake)在 isinstance 上一夜之间不合格。"""
+        from polygateway.ports import OcrTransport
+
+        members = {
+            name
+            for name in vars(OcrTransport)
+            if not name.startswith("_") and callable(getattr(OcrTransport, name))
+        }
+        assert members == {"recognize_text", "parse_layout", "check_health"}
+
     def test_missing_method_rejected(self):
         from polygateway.ports import OcrTransport