feat(sdk): add Python SDK with QUIC and FFI transport backends

Implements quicproquo-py with two transport backends:
- Async QUIC transport via aioquic with v2 protobuf wire format
- Synchronous Rust FFI transport via CFFI wrapping libquicproquo_ffi

Includes manual protobuf encode/decode (no codegen), full RPC coverage
(auth, delivery, channels, users, keys, health), PyPI-ready packaging,
async echo bot and FFI demo examples, and 15 passing unit tests.

sdks/python/examples/ffi_demo.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+"""Example: synchronous messaging using the Rust FFI backend.
+
+Requires libquicproquo_ffi to be built:
+    cargo build --release -p quicproquo-ffi
+
+Set QPQ_LIB_PATH if the library is not in the default search path.
+
+Usage:
+    python ffi_demo.py --server 127.0.0.1:5001 \
+        --ca-cert ca.pem --user alice --pass secret
+"""
+
+from __future__ import annotations
+
+import argparse
+
+from quicproquo import QpqClient, ConnectOptions
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description="qpq FFI demo")
+    parser.add_argument("--server", default="127.0.0.1:5001")
+    parser.add_argument("--ca-cert", default="ca.pem")
+    parser.add_argument("--server-name", default="")
+    parser.add_argument("--user", required=True)
+    parser.add_argument("--pass", dest="password", required=True)
+    parser.add_argument("--recipient", required=True, help="recipient username")
+    parser.add_argument("--message", default="hello from Python SDK!")
+    args = parser.parse_args()
+
+    opts = ConnectOptions(
+        addr=args.server,
+        ca_cert_path=args.ca_cert,
+        server_name=args.server_name,
+    )
+
+    client = QpqClient.connect_ffi(opts)
+
+    try:
+        print(f"Connected to {args.server}")
+
+        client.ffi_login(args.user, args.password)
+        print(f"Logged in as {args.user}")
+
+        client.ffi_send(args.recipient, args.message.encode("utf-8"))
+        print(f"Sent message to {args.recipient}")
+
+        print("Waiting for messages (5s)...")
+        messages = client.ffi_receive(timeout_ms=5000)
+        for msg in messages:
+            print(f"  received: {msg}")
+
+    finally:
+        client.close_sync()
+        print("Disconnected.")
+
+
+if __name__ == "__main__":
+    main()