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/ports.py

@@ -194,6 +194,8 @@ class OutcomeAwareSelector(Protocol):
 
     def record_outcome(self, source_name: str, ok: bool) -> None: ...
 
+    def health(self, source_name: str) -> float: ...
+
 
 @runtime_checkable
 class StructuredOutputStrategy(Protocol):