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/sources.py

@@ -7,6 +7,7 @@ RetryMW 的进程本地状态——熔断开路的源在本地记冷却截止,
 
 from __future__ import annotations
 
+import random
 import time
 from typing import TYPE_CHECKING
 
@@ -15,6 +16,9 @@ if TYPE_CHECKING:
 
     from polygateway.types import SourceConfig, SourceStats
 
+_EWMA_ALPHA = 0.2  # 健康 EWMA 步长: 约 10 次成功从谷底爬回 0.9(天然 slow-start)
+_SCORE_FLOOR = 0.05  # 探索地板: 塌陷源保有微量被选概率,恢复靠真实成功自证
+
 
 class RoundRobinSelector:
     """轮转起点后移(CHS selector.py:20 同款);单 client 内游标推进。"""
@@ -41,6 +45,43 @@ class LeastInflightSelector:
         return sorted(sources, key=lambda s: stats[s.name].inflight if s.name in stats else 0)
 
 
+class HealthAwareSelector:
+    """健康感知选源(M2.5 设计 §3.2;蓝本 Envoy least-request + gRPC WRR)。
+
+    score = max(ewma_success, 地板) / (1 + inflight)。头名经 P2C(随机取
+    两源比分,高者先)引入探索;其余按分数降序。健康态为进程本地(业界
+    共识: Envoy/Finagle/gRPC 全本地),属 client 实例,不违反纯 asyncio 中立。
+    已知取舍(设计 §3.2): 仅头名随机化,多 worker 溢出会集中到同一次优源。
+    """
+
+    def __init__(self, *, rng: Callable[[], float] = random.random) -> None:
+        self._rng = rng
+        self._ewma: dict[str, float] = {}
+
+    def record_outcome(self, source_name: str, ok: bool) -> None:
+        """尝试结果喂数(OutcomeAwareSelector 端口);初始 1.0 乐观起步。"""
+        prev = self._ewma.get(source_name, 1.0)
+        self._ewma[source_name] = prev + _EWMA_ALPHA * ((1.0 if ok else 0.0) - prev)
+
+    def _score(self, name: str, stats: dict[str, SourceStats]) -> float:
+        ewma = max(self._ewma.get(name, 1.0), _SCORE_FLOOR)
+        inflight = stats[name].inflight if name in stats else 0
+        return ewma / (1.0 + inflight)
+
+    def order(
+        self, sources: list[SourceConfig], stats: dict[str, SourceStats]
+    ) -> list[SourceConfig]:
+        if len(sources) < 2:
+            return list(sources)
+        ranked = sorted(sources, key=lambda s: self._score(s.name, stats), reverse=True)
+        # P2C: 随机取两源比分,胜者提为头名(平分取采样序首位)
+        i = int(self._rng() * len(sources)) % len(sources)
+        j = int(self._rng() * len(sources)) % len(sources)
+        a, b = sources[i], sources[j]
+        head = a if self._score(a.name, stats) >= self._score(b.name, stats) else b
+        return [head] + [s for s in ranked if s.name != head.name]
+
+
 class SourceCooldownMemo:
     """进程本地的源冷却备忘(CHS governance.py:107 同款)。