feat: implement Client crate and C FFI bindings (#73)

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

.github/workflows/ci.yml

@@ -36,3 +36,21 @@ jobs:
       - run: rustup update stable && rustup default stable
       - run: rustup component add rustfmt
       - run: cargo fmt --all -- --check
+
+  c-ffi-smoketest:
+    name: C FFI Smoketest
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - run: rustup update stable && rustup default stable
+      - name: Install valgrind
+        run: sudo apt-get install -y valgrind
+      - name: Build C FFI example
+        run: make
+        working-directory: crates/client-ffi/examples/message-exchange
+      - name: Run C FFI smoketest
+        run: ./c-client
+        working-directory: crates/client-ffi/examples/message-exchange
+      - name: Run C FFI smoketest under valgrind
+        run: make valgrind
+        working-directory: crates/client-ffi/examples/message-exchange