feat: Sprint 3 — C FFI bindings, WASM compilation, Python example, SDK docs

- Create quicproquo-ffi crate with 7 extern "C" functions: connect,
  login, send, receive, disconnect, last_error, free_string
  (produces libquicproquo_ffi.so and .a)
- Feature-gate quicproquo-core for WASM: identity, hybrid_kem,
  safety_numbers, sealed_sender, app_message, padding, transcript
  all compile to wasm32-unknown-unknown
- Add Python ctypes example (examples/python/qpq_client.py) with
  QpqClient wrapper class and CLI
- Add SDK documentation: FFI reference, WASM guide, qpq-gen generators
- Update Dockerfile for quicproquo-ffi workspace member

Cargo.toml

@@ -11,6 +11,7 @@ members = [
     "crates/quicproquo-gen",
     "crates/quicproquo-gui",
     "crates/quicproquo-mobile",
+    "crates/quicproquo-ffi",
     # P2P crate uses iroh (~90 extra deps). Kept in the workspace so it can be
     # referenced as an optional dependency; only compiled when the `mesh` feature
     # is enabled on quicproquo-client.