Implement a `client` crate that wraps the `libchat` context behind a
simple `ChatClient<D>` API. The delivery strategy is pluggable via a
`DeliveryService` trait, with two implementations provided:
- `InProcessDelivery` — shared `MessageBus` for single-process tests
- `CDelivery` — C function-pointer callback for the FFI layer
Add a `client-ffi` crate that exposes the client as a C API via
`safer-ffi`. A `generate-headers` binary produces the companion C
header.
Include two runnable examples:
- `examples/in-process` — Alice/Bob exchange using in-process delivery
- `examples/c-ffi` — same exchange written entirely in C; smoketested
under valgrind (to catch memory leaks) in CI
iterates: #71
