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

docker/Dockerfile

@@ -23,6 +23,7 @@ COPY crates/quicproquo-kt/Cargo.toml         crates/quicproquo-kt/Cargo.toml
 COPY crates/quicproquo-plugin-api/Cargo.toml crates/quicproquo-plugin-api/Cargo.toml
 COPY crates/quicproquo-gui/Cargo.toml        crates/quicproquo-gui/Cargo.toml
 COPY crates/quicproquo-mobile/Cargo.toml     crates/quicproquo-mobile/Cargo.toml
+COPY crates/quicproquo-ffi/Cargo.toml        crates/quicproquo-ffi/Cargo.toml
 
 # Create dummy source files so `cargo build` can resolve the dependency graph
 # and cache the compiled dependencies before copying real source.
@@ -38,6 +39,7 @@ RUN mkdir -p \
         crates/quicproquo-plugin-api/src \
         crates/quicproquo-gui/src \
         crates/quicproquo-mobile/src \
+        crates/quicproquo-ffi/src \
     && echo 'fn main() {}' > crates/quicproquo-server/src/main.rs \
     && echo 'fn main() {}' > crates/quicproquo-client/src/main.rs \
     && echo 'fn main() {}' > crates/quicproquo-gen/src/main.rs \
@@ -48,7 +50,8 @@ RUN mkdir -p \
     && touch crates/quicproquo-kt/src/lib.rs \
     && touch crates/quicproquo-plugin-api/src/lib.rs \
     && touch crates/quicproquo-gui/src/lib.rs \
-    && touch crates/quicproquo-mobile/src/lib.rs
+    && touch crates/quicproquo-mobile/src/lib.rs \
+    && touch crates/quicproquo-ffi/src/lib.rs
 
 # Schemas must exist before the proto crate's build.rs runs.
 COPY schemas/ schemas/