fix: address M2.5 verifier findings before merge

AIMD ceiling now respects per-source max_concurrency and the pacer is
assembled explicitly in the client; MIN_CALLS parses as strict int;
acceptance doc corrects source-5 attempt count to 549; design and
migration notes aligned with implemented 429/stall/suppression
semantics and AIMD constants documented.

src/polygateway/config.py

@@ -231,7 +231,8 @@ def _load_breaker(
         # 派生规则: 探针租约须撑过一次最慢调用,且不短于冷却期(第三项保证守卫恒成立)
         probe_ttl_s = max(2 * slowest, cooldown_s, probe_floor)
     # M2.5 失败率通道参数(可选键,库缺省——韧性参数缺省先例同 backpressure)
-    min_calls = int(_opt_float(env, f"{scope}__BREAKER__MIN_CALLS", 10))
+    raw_min = env.get(f"{scope}__BREAKER__MIN_CALLS")
+    min_calls = int(_cast(raw_min, "int", f"{scope}__BREAKER__MIN_CALLS")) if raw_min else 10
     fail_rate = _opt_float(env, f"{scope}__BREAKER__FAIL_RATE", 0.6)
     window_s = _opt_float(env, f"{scope}__BREAKER__WINDOW_S", 60.0)
     max_cooldown_s = _opt_float(env, f"{scope}__BREAKER__MAX_COOLDOWN_S", max(300.0, cooldown_s))