feat: add health-aware P2C selector as default

Score is success-rate EWMA over (1 + inflight) with a 0.05 exploration
floor so quarantined sources can prove recovery; EWMA climb doubles as
slow-start. New optional OutcomeAwareSelector port feeds attempt
outcomes.

src/polygateway/ports.py

@@ -181,6 +181,18 @@ class SourceSelector(Protocol):
     ) -> list[SourceConfig]: ...
 
 
+@runtime_checkable
+class OutcomeAwareSelector(Protocol):
+    """可选选源器扩展(M2.5 设计 §3.2): 消费尝试结果以维护健康视图。
+
+    RetryMW 构造时 isinstance 判定一次;非本 Protocol 的选源器不受影响。
+    喂数口径: 真实成功 ok=True;Transient/SourceDead/429 ok=False;
+    ResultInvalid 与"网关健康拒坏请求"不喂(坏结果 ≠ 坏服务)。
+    """
+
+    def record_outcome(self, source_name: str, ok: bool) -> None: ...
+
+
 @runtime_checkable
 class StructuredOutputStrategy(Protocol):
     """结构化输出策略(D7/D14): 请求侧叠加 + 响应侧解析。"""