The lazy store embedded appservice.StateStore, but the client's store is
also type-asserted to crypto.StateStore -- a wider interface -- and
NewCryptoHelper refuses to start when that assertion fails. Embedding the
narrower interface dropped GetEncryptionEvent and FindSharedRooms from the
concrete type, so the bot died on startup with "the client state store
must implement crypto.StateStore".
Embed both, assert both at compile time, and resolve GetEncryptionEvent
lazily like IsEncrypted since the crypto machine reads megolm rotation
settings through it.
Without /sync nothing backfills the state store, so a room the bot had
not yet heard from looked unencrypted -- IsEncrypted answers false with
no error -- and any message the bot started on its own (ambient DMs,
expedition beats, briefings) went out in the clear. Under /sync the
cryptohelper registered StateStoreSyncHandler and the initial sync
carried every joined room's state, so the store was warm before the bot
ever spoke; the appservice port replicated the crypto plumbing but not
that backfill.
Resolve encryption and membership from the server on first read instead,
and refuse the send when they cannot be resolved: a message that fails is
recoverable, a plaintext one that has landed is not. Members are resolved
the same way, since an unfetched room shares the group session with
nobody.
