fix: address independent verification findings

Classify empty completions as transient per human ruling (fixes flaky
real-gateway smoke and prevents caching empty responses), rename the
factory injection parameter gate to breaker per the frozen design,
rewrite the probe-entry cleanup without except BaseException, declare
python-dotenv explicitly, add a mid-backoff cancellation test, and
record all implementation errata in the design and architecture docs.

src/polygateway/backends/memory/limiter.py

@@ -1,7 +1,8 @@
 """进程内限流后端: 与 Redis 版同一契约的六道闸实现(D3 双后端)。
 
 语义蓝本 CHS `app/coordination/limiter.py`: 并发 = 带 TTL 的租约(持有者
-死亡后过期回收);RPM/TPM = 分钟滑动窗口计数;TPM 入场按 est 预扣,settle
+死亡后过期回收);RPM/TPM = 分钟**固定窗口**计数(`int(now/60)`,与 CHS
+Lua 同款口径);TPM 入场按 est 预扣,settle
 按实际结算多退少补且退款落 acquire 时的窗口。检查-占用在单次同步段内完成
 (无 await 穿插),单事件循环下天然原子;本实现不跨进程,是单进程部署的
 正确答案(多 worker 用 M2 Redis 后端)。