Requests missing the cache for the same key at once share one read and one allowance

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -45,6 +45,10 @@ APIs change between minors without a deprecation period.
   an endpoint it does not measure as a queue, so an external model (OpenAI, another llmgw cluster) was held to what a
   saturation of 0.85 allows of "in flight / 4"; the rest waited and timed out with 504. An outside provider queues for
   itself and says 429 when full, which its breaker already hears: it no longer counts towards saturation.
+- **A valid key told to slow down under load, every thirty seconds.** When a busy key's cached entry expired, every
+  request in flight missed the cache together and each took one of the address's allowance for unknown keys: past a
+  hundred or so streams at once, a valid key got 429 "too many unrecognised api keys". Requests missing the same key
+  at once now share one read and one allowance.
 
 ## v0.16.1 — 2026-10-30