Consolidates the persisted-plaintext-decrypt-cache pattern (already used by watch
history) into a shared factory, persistedPlaintextCache.js, and adds a DM cache
on top of it. watchHistoryDecryptCache now delegates to the factory (same public
API; tests unchanged).
decryptDirectMessageEvent now also consults/writes a persisted cache keyed by the
immutable DM event id (JSON of the decrypt result; flushed once per
listDirectMessages batch; cleared on logout alongside the in-memory cache). DM
events never change, so a reload re-decrypts only messages it has not seen and
skips the (serialized, slow) nip-07 extension for the rest.
Harness (dm-repro, now cold/warm): 20 DMs -> COLD nip04.decrypt 50, WARM 0 —
reloads cost zero extension decrypt calls. Also excludes the actor's own key from
NIP-04 remote candidates (decrypting against self is always wasted). DM unit +
integration and watch-history cache tests green.
Co-Authored-By: Claude Opus 4.8 <[email protected]>