feat: add rate-channel breaker config and health_aware default

Threshold auto-raise now uses per-source concurrency only (the M2
global-concurrency formula neutered the breaker at scale). Four new
optional keys: MIN_CALLS, FAIL_RATE, WINDOW_S, MAX_COOLDOWN_S.

src/polygateway/types.py

@@ -152,15 +152,28 @@ class RetryPolicy:
 
 @dataclass(frozen=True)
 class BreakerConfig:
-    """熔断配置;probe_ttl_s 是半开探针租约时长(持有者死亡后自动回收)。"""
+    """熔断配置;probe_ttl_s 是半开探针租约时长(持有者死亡后自动回收)。
+
+    M2.5 双通道: fail_threshold 是连续失败通道;min_calls/fail_rate/window_s
+    是失败率通道(窗口样本 ≥ min_calls 且失败率 ≥ fail_rate 即开路,429 不入);
+    开路时长按重开次数指数递增,封顶 max_cooldown_s(设计 2026-07-21-m25)。
+    """
 
     fail_threshold: int
     cooldown_s: float
     probe_ttl_s: float
+    min_calls: int = 10
+    fail_rate: float = 0.6
+    window_s: float = 60.0
+    max_cooldown_s: float = 300.0
 
     def __post_init__(self) -> None:
         if self.fail_threshold < 1 or self.cooldown_s <= 0 or self.probe_ttl_s <= 0:
             raise ValueError("熔断配置要求 fail_threshold ≥ 1 且 cooldown_s/probe_ttl_s > 0")
+        if self.min_calls < 1 or not (0.0 < self.fail_rate <= 1.0) or self.window_s <= 0:
+            raise ValueError("失败率通道要求 min_calls ≥ 1、0 < fail_rate ≤ 1、window_s > 0")
+        if self.max_cooldown_s < self.cooldown_s:
+            raise ValueError("max_cooldown_s 不得小于 cooldown_s(退避封顶低于初值)")
 
 
 @dataclass(frozen=True)