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.lock

@@ -5481,6 +5481,7 @@ dependencies = [
  "ciborium",
  "criterion",
  "ed25519-dalek 2.2.0",
+ "getrandom 0.2.17",
  "hkdf",
  "hmac",
  "ml-kem",
@@ -5501,6 +5502,17 @@ dependencies = [
  "zeroize",
 ]
 
+[[package]]
+name = "quicproquo-ffi"
+version = "0.1.0"
+dependencies = [
+ "anyhow",
+ "hex",
+ "quicproquo-client",
+ "serde_json",
+ "tokio",
+]
+
 [[package]]
 name = "quicproquo-gen"
 version = "0.1.0"