feat: carry the per-call tier down to the transport that must send it

The Transport port took the request apart into five arguments, so a tier
placed on ChatRequest could never reach _build_payload: the field was set,
read by nobody, and silently ignored - the exact shape of failure that sent
downstream to extra_body in the first place.

complete() now takes reasoning_effort with no default, matching the
TelemetryRecorder convention: a default would turn a missing hand-off into
a silent 'no opinion'. All four fakes move with it, since @runtime_checkable
checks method names and not signatures.

EmbeddingTransport and OcrTransport are deliberately left alone - they have
no reasoning semantics - and a test now holds that line.

_build_payload drops its inline sugar conversion for effective_effort(), so
the guard and the hot path share one judgement, and passes the source's
effort_fallback for the same reason.

tests/unit/test_ports.py

@@ -1,15 +1,18 @@
 """ports.py 端口冻结测试(M1 设计 §4): Protocol 结构性检查 + Gate 快照校验。"""
 
+import inspect
 from typing import Any
 
 import pytest
 
 from polygateway.ports import (
     CacheBackend,
+    EmbeddingTransport,
     GateDecision,
     GateState,
     GateUpdate,
     Middleware,
+    OcrTransport,
     Permit,
     ProviderGate,
     RateLimiter,
@@ -69,7 +72,7 @@ class _DummyMw:
 
 
 class _DummyTransport:
-    async def complete(self, *, messages, source, stream, overlay, call_id):
+    async def complete(self, *, messages, source, stream, overlay, call_id, reasoning_effort):
         raise NotImplementedError
 
 
@@ -142,6 +145,34 @@ def test_protocols_are_runtime_checkable(impl, protocol):
     assert isinstance(impl, protocol)
 
 
+class TestReasoningTierIsOnlyOnTheChatPort:
+    """档位属于 chat 端口,且**只属于**它(Task 5b)。
+
+    `@runtime_checkable` 只查方法名不查签名,故协议签名本身必须被显式断言——
+    否则实现漏改一个参数,要到运行期调用才会以 `TypeError` 现形,而那时的现场
+    离根因已经很远。
+    """
+
+    def test_chat_transport_carries_the_per_call_tier(self):
+        params = inspect.signature(Transport.complete).parameters
+        assert "reasoning_effort" in params
+        # 不给默认值是有意的(与 TelemetryRecorder 同一既有约定): 库外无第三方
+        # 实现者,写全签名成本为零,而默认值会把"漏传"变成静默的"不表态"
+        assert params["reasoning_effort"].default is inspect.Parameter.empty
+
+    @pytest.mark.parametrize(
+        ("protocol", "method"),
+        [
+            (EmbeddingTransport, "embed"),
+            (OcrTransport, "recognize_text"),
+            (OcrTransport, "parse_layout"),
+        ],
+    )
+    def test_other_transports_have_no_reasoning_tier(self, protocol, method):
+        """embedding 与 OCR 没有推理语义,给它们加档位只会静默无效(issue #4 同款决策)。"""
+        assert "reasoning_effort" not in inspect.signature(getattr(protocol, method)).parameters
+
+
 class _DummyStatusProvider(_DummyRecorder):
     @property
     def telemetry_status(self) -> TelemetryStatus: