Multiple subscribers (history list + notifications) decrypt the same DM event
at once; before the in-memory cache fills, each hit the serialized nip-07
extension, multiplying round-trips per message. Cache the in-flight decrypt
promise by event id so concurrent callers share one decrypt.
Harness: cold-load extension decrypt calls drop from ~50-60 to exactly 20 for 20
DMs (1 per message); warm reload stays 0 via the persisted cache. Combined with
the earlier ordering/sequential and persist-cache fixes, legacy DM decryption
went from ~241 extension calls (20 DMs) to 20 cold / 0 warm.
Co-Authored-By: Claude Opus 4.8 <[email protected]>
