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/client.py

@@ -24,7 +24,12 @@ from polygateway.middleware.structured import StructuredMW
 from polygateway.middleware.telemetry import TelemetryEmitter, TelemetryMW
 from polygateway.pricing import PricingTable
 from polygateway.providers import get_provider
-from polygateway.sources import LeastInflightSelector, RoundRobinSelector, SourceCooldownMemo
+from polygateway.sources import (
+    HealthAwareSelector,
+    LeastInflightSelector,
+    RoundRobinSelector,
+    SourceCooldownMemo,
+)
 from polygateway.transports.openai_compat import OpenAICompatTransport
 from polygateway.types import ChatRequest, LLMResponse
 
@@ -193,6 +198,7 @@ class GatewayClient:
         cache: CacheBackend | None = None,
         telemetry: TelemetryRecorder | None = None,
         registry: Mapping[str, ProviderProfile] | None = None,
+        rng: Any = random.random,
     ) -> GatewayClient:
         """按配置装配;显式传入的后端实例即共享(None 项按配置自建私有实例)。"""
         sources = list(settings.sources)
@@ -201,7 +207,7 @@ class GatewayClient:
         return cls(
             scope=settings.scope,
             sources=sources,
-            selector=_build_selector(settings.selector),
+            selector=_build_selector(settings.selector, rng=rng),
             limiter=limiter or _build_limiter(settings, sources),
             breaker=breaker or _build_breaker(settings),
             transport=OpenAICompatTransport(registry=registry),
@@ -272,8 +278,12 @@ def _build_breaker(settings: GatewaySettings) -> ProviderGate:
     return InMemoryGate(config=settings.breaker)
 
 
-def _build_selector(name: str) -> SourceSelector:
-    return RoundRobinSelector() if name == "round_robin" else LeastInflightSelector()
+def _build_selector(name: str, *, rng: Any = random.random) -> SourceSelector:
+    if name == "round_robin":
+        return RoundRobinSelector()
+    if name == "least_inflight":
+        return LeastInflightSelector()
+    return HealthAwareSelector(rng=rng)
 
 
 def _build_cache(settings: GatewaySettings) -> CacheBackend | None: