feat: gate in-call demotion on credible alternative health

Round 3 showed unconditional yield-after-two-failures pushes the third
attempt onto known-bad sources in heterogeneous pools (83% vs 10%
expected success). OutcomeAwareSelector now exposes health(); a failed
source only yields when some untried candidate scores at least half its
health. Health-blind selectors keep the unconditional rule.

src/polygateway/sources.py

@@ -63,6 +63,10 @@ class HealthAwareSelector:
         prev = self._ewma.get(source_name, 1.0)
         self._ewma[source_name] = prev + _EWMA_ALPHA * ((1.0 if ok else 0.0) - prev)
 
+    def health(self, source_name: str) -> float:
+        """EWMA 裸值(OutcomeAwareSelector 端口): RetryMW 降权门槛用。"""
+        return self._ewma.get(source_name, 1.0)
+
     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