Tracks active Streamable HTTP sessions for a server mount.
Produce a cryptographically-secure, hex-encoded 256-bit session id.
A string-keyed cache of V with a per-entry insertion/activity timestamp, bounded by an idle TTL and a maximum live-entry count. It owns the value map and the parallel timestamp map together so the "remove from both" invariant lives in one place, and runs the TTL sweep + LRU cap eviction internally.