Implements SharedHttpClient (ArcSwap<ClientWithMiddleware>) with HttpClientConfig
(pool/timeout/retry/optional CA bundle+client cert), RetryTransientMiddleware from
reqwest-retry, and inlined RetryAfterMiddleware (~90 lines, bounded HashMap with LRU
eviction, parses Retry-After seconds + HTTP-date, sleeps on 429/503). reload() via
ArcSwap. No env-var reads; per-request credential injection only. 24 unit tests.