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.

tests/unit/test_health_selector.py

@@ -69,6 +69,13 @@ class TestHealthAwareSelector:
         assert order[0].name == "s1"  # 两候选中 s1 胜出
         assert order[-1].name == "s3"  # 塌陷源垫底
 
+    def test_health_exposes_raw_ewma(self):
+        sel = HealthAwareSelector(rng=lambda: 0.0)
+        assert sel.health("s1") == pytest.approx(1.0)  # 乐观初始
+        for _ in range(3):
+            sel.record_outcome("s1", ok=False)
+        assert sel.health("s1") == pytest.approx(1.0 * 0.8**3)
+
     def test_missing_stats_defaults_to_zero_inflight(self):
         sel = HealthAwareSelector(rng=lambda: 0.0)
         assert [s.name for s in sel.order([_src("s1")], {})] == ["s1"]