Make the conversations core Send so the threaded client can own it behind an
Arc<Mutex<Core>>: a background worker polls the transport and handles inbound
payloads while the application thread issues outbound calls (send, create
conversation). Sharing the core across those two threads means moving it into
the spawned worker, which is only legal if it is Send. Access stays serialized
by the client's Mutex (one thread at a time), so the core needs Send but not
Sync and carries no lock of its own. See
docs/adr/0001-client-event-system.md for the background-poller design.
The Rc<RefCell> service-sharing is what made the core !Send. Context is de-Rc'd
and renamed to Core, owning its services outright and driving the inbox and
conversation primitives with plain &mut self.
- Services (identity, delivery, store, registry, MLS context, causal history)
are bundled into a ServiceContext<S> behind an ExternalServices trait, with
S = (DS, RS, CS). Constructors live on the (DS, RS, CS) form because S cannot
be inferred backwards through S::DS.
- Inbox, InboxV2, PrivateV1Convo, and GroupV1Convo become non-generic and
receive the ServiceContext bundle as a &mut/& parameter; no Rc or
RefCell-as-shared-state remains, so Core is Send whenever its injected
services are.
- Dispatch branches on ConversationKind in one place: Core rebuilds the target
as a Convo<S>/GroupConvo<S> trait object bound to the service bundle, so
conversations never escape the orchestrator.
- CausalHistoryStore drops its Rc, keeping a plain RefCell.
