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

crates/quicproquo-core/src/error.rs

@@ -6,6 +6,7 @@ use thiserror::Error;
 #[derive(Debug, Error)]
 pub enum CoreError {
     /// Cap'n Proto serialisation or deserialisation failed.
+    #[cfg(feature = "native")]
     #[error("Cap'n Proto error: {0}")]
     Capnp(#[from] capnp::Error),
 

crates/quicproquo-core/src/identity.rs

@@ -17,8 +17,6 @@
 //! collision-resistant identifier for logging.
 
 use ed25519_dalek::{Signer as DalekSigner, SigningKey, VerifyingKey};
-use openmls_traits::signatures::Signer;
-use openmls_traits::types::{Error as MlsError, SignatureScheme};
 use serde::{Deserialize, Serialize};
 use sha2::{Digest, Sha256};
 use zeroize::Zeroizing;
@@ -87,15 +85,16 @@ impl IdentityKeypair {
 /// Implement the openmls `Signer` trait so `IdentityKeypair` can be passed
 /// directly to `KeyPackage::builder().build(...)` without needing the external
 /// `openmls_basic_credential` crate.
-impl Signer for IdentityKeypair {
-    fn sign(&self, payload: &[u8]) -> Result<Vec<u8>, MlsError> {
+#[cfg(feature = "native")]
+impl openmls_traits::signatures::Signer for IdentityKeypair {
+    fn sign(&self, payload: &[u8]) -> Result<Vec<u8>, openmls_traits::types::Error> {
         let sk = self.signing_key();
         let sig: ed25519_dalek::Signature = sk.sign(payload);
         Ok(sig.to_bytes().to_vec())
     }
 
-    fn signature_scheme(&self) -> SignatureScheme {
-        SignatureScheme::ED25519
+    fn signature_scheme(&self) -> openmls_traits::types::SignatureScheme {
+        openmls_traits::types::SignatureScheme::ED25519
     }
 }
 

crates/quicproquo-core/src/lib.rs

@@ -1,6 +1,28 @@
 //! Core cryptographic primitives, MLS group state machine, and hybrid
 //! post-quantum KEM for quicproquo.
 //!
+//! # WASM support
+//!
+//! When compiled with `--no-default-features` (disabling the `native` feature),
+//! the following modules are available for `wasm32-unknown-unknown`:
+//!
+//! - `identity` — Ed25519 identity keypair (generate, sign, verify)
+//! - `hybrid_kem` — X25519 + ML-KEM-768 hybrid key encapsulation
+//! - `safety_numbers` — Signal-style safety number computation
+//! - `sealed_sender` — sender identity + Ed25519 signature envelope
+//! - `app_message` — rich application message serialisation/parsing
+//! - `padding` — message padding to hide plaintext lengths
+//! - `transcript` — encrypted tamper-evident message transcript
+//! - `error` — `CoreError` type
+//!
+//! The following modules require the `native` feature (MLS, OPAQUE, Cap'n Proto):
+//!
+//! - `group` — MLS group state machine (openmls)
+//! - `keypackage` — MLS KeyPackage generation
+//! - `hybrid_crypto` — hybrid HPKE provider for OpenMLS
+//! - `keystore` — OpenMLS key store with optional disk persistence
+//! - `opaque_auth` — OPAQUE cipher suite configuration
+//!
 //! # Module layout
 //!
 //! | Module        | Responsibility                                                   |
@@ -15,36 +37,50 @@
 
 mod app_message;
 mod error;
-mod group;
-mod hybrid_crypto;
 mod hybrid_kem;
 mod identity;
-mod keypackage;
-mod keystore;
-pub mod opaque_auth;
 pub mod padding;
 pub mod safety_numbers;
 pub mod sealed_sender;
 pub mod transcript;
 
-// ── Public API ────────────────────────────────────────────────────────────────
+// ── Native-only modules (MLS, OPAQUE, filesystem) ───────────────────────────
+#[cfg(feature = "native")]
+mod group;
+#[cfg(feature = "native")]
+mod hybrid_crypto;
+#[cfg(feature = "native")]
+mod keypackage;
+#[cfg(feature = "native")]
+mod keystore;
+#[cfg(feature = "native")]
+pub mod opaque_auth;
+
+// ── Public API (always available) ───────────────────────────────────────────
 
 pub use app_message::{
     serialize, serialize_chat, serialize_reaction, serialize_read_receipt, serialize_reply,
     serialize_typing, parse, generate_message_id, AppMessage, MessageType, VERSION as APP_MESSAGE_VERSION,
 };
 pub use error::CoreError;
-pub use group::{GroupMember, ReceivedMessage, ReceivedMessageWithSender};
 pub use hybrid_kem::{
     hybrid_decrypt, hybrid_encrypt, HybridKemError, HybridKeypair, HybridKeypairBytes,
     HybridPublicKey,
 };
-pub use hybrid_crypto::{HybridCrypto, HybridCryptoProvider};
 pub use identity::{verify_delivery_proof, IdentityKeypair};
-pub use keypackage::{generate_key_package, validate_keypackage_ciphersuite};
-pub use keystore::DiskKeyStore;
 pub use safety_numbers::compute_safety_number;
 pub use transcript::{
     read_transcript, verify_transcript_chain, ChainVerdict, DecodedRecord, TranscriptRecord,
     TranscriptWriter,
 };
+
+// ── Public API (native only) ────────────────────────────────────────────────
+
+#[cfg(feature = "native")]
+pub use group::{GroupMember, ReceivedMessage, ReceivedMessageWithSender};
+#[cfg(feature = "native")]
+pub use hybrid_crypto::{HybridCrypto, HybridCryptoProvider};
+#[cfg(feature = "native")]
+pub use keypackage::{generate_key_package, validate_keypackage_ciphersuite};
+#[cfg(feature = "native")]
+pub use keystore::DiskKeyStore;