fix: stop telling people the cache key records the mapped tier

It records the asked-for one. CacheMW sits outside the transport in the
onion, so at lookup time the nearest-mapping has not happened yet and the
applied tier does not exist. Telemetry's success rows do record the mapped
tier, which is where the confusion came from — the warning conflated the
two and would have sent anyone debugging a cache miss the wrong way.

Also repairs the design doc: the 2026-09-05 rollback note had been spliced
into the equivalence table, orphaning its last row, and §3.1 still said
seven tiers after `auto` made it eight.

src/polygateway/thinking.py

@@ -556,7 +556,8 @@ def _settle_tier(
         )
     logger.warning(
         "模型 {} 不支持 reasoning_effort={},按 effort_fallback=nearest 改用最近的 {} 档;"
-        "本次真正发出去的是后者,遥测与缓存 key 记的也是后者",
+        "本次真正发出去的、以及遥测成功行记的都是后者,但**缓存 key 记的是前者**"
+        "(CacheMW 在洋葱里比 transport 更外,查缓存时映射尚未发生,拿不到实发档)",
         model,
         effort.value,
         mapped.value,