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/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
     }
 }