docs: record approval, verification evidence and a follow-up gap

Corrects the changelog claim that the old guard messages only named env keys:
they named fields too, it was the remediation advice that pointed at env keys.

Records the human approval of the design, the mutation-testing evidence from
the independent verifier, and the same-class gap it found: 14 checks (redis_url
presence, telemetry paths, enum validity) still live only in from_env, while
client.py asserts they were already validated. Deliberately out of scope here.

CHANGELOG.md

@@ -11,7 +11,7 @@
 
 直接构造 `GatewaySettings` 或对它做 `dataclasses.replace` 时,若上述组合非法,**现在会在构造期抛 `ValueError`**,而不是留到运行时。经 `from_env()` 装配的调用方**不受影响**——那条路本就跑这些守卫。手工拼配置(如从 YAML 读出后构造)的调用方若此前撞上过上述任一故障,升级后会在启动时立即得到点名字段的报错。
 
-守卫的报错文案改为点字段名(`lease_ttl_s`、`backpressure.stall_window_s`、`breaker.probe_ttl_s`)。原文案只点环境变量键,而不走 env 的调用方从没设过那些键。键名映射见 `.env.example` 与 wiki `参考-配置键`。
+守卫报错文案的**补救建议**改为点字段名(`lease_ttl_s`、`backpressure.stall_window_s`、`breaker.probe_ttl_s`)。原文案已点出字段名,但建议部分给的是环境变量键(如"调大 `PGW_LEASE_TTL_S`"),而不走 env 的调用方从没设过那些键。键名映射见 `.env.example` 与 wiki `参考-配置键`。
 
 ## 1.0.0(2026-07-22)