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/bot.py

@@ -0,0 +1,96 @@
+#!/usr/bin/env python3
+"""Example: async echo bot using the quicproquo Python SDK.
+
+Connects to a qpq server, authenticates, and echoes back any received
+messages with a "[bot] " prefix.
+
+Usage:
+    python bot.py --server 127.0.0.1:5001 --ca-cert ca.pem
+
+This example uses the QUIC transport with the v2 wire format.
+OPAQUE authentication requires external crypto; this demo assumes
+a session token is obtained externally and set via --token.
+"""
+
+from __future__ import annotations
+
+import argparse
+import asyncio
+import signal
+import sys
+
+from quicproquo import QpqClient, ConnectOptions
+
+
+async def run_bot(opts: ConnectOptions, token: bytes, identity_key: bytes) -> None:
+    client = await QpqClient.connect(opts)
+    client.set_session_token(token)
+
+    print(f"Connected to {opts.addr}")
+
+    health = await client.health()
+    print(f"Server status: {health.status} (v{health.version})")
+
+    # Poll loop.
+    running = True
+
+    def on_signal() -> None:
+        nonlocal running
+        running = False
+        print("\nShutting down...")
+
+    loop = asyncio.get_running_loop()
+    for sig in (signal.SIGINT, signal.SIGTERM):
+        loop.add_signal_handler(sig, on_signal)
+
+    while running:
+        try:
+            messages = await client.receive_wait(
+                identity_key, timeout_ms=5000
+            )
+        except Exception as exc:
+            print(f"receive error: {exc}")
+            await asyncio.sleep(1)
+            continue
+
+        for msg in messages:
+            text = msg.data.decode("utf-8", errors="replace")
+            print(f"[seq={msg.seq}] {text}")
+
+            # Echo back with prefix.
+            echo = f"[bot] {text}".encode("utf-8")
+            try:
+                seq, _ = await client.send(identity_key, echo)
+                print(f"  -> echoed (seq={seq})")
+            except Exception as exc:
+                print(f"  -> send error: {exc}")
+
+    await client.close()
+    print("Disconnected.")
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description="qpq echo bot")
+    parser.add_argument("--server", default="127.0.0.1:5001", help="server address")
+    parser.add_argument("--ca-cert", default="", help="CA certificate path")
+    parser.add_argument("--server-name", default="", help="TLS server name")
+    parser.add_argument("--token", required=True, help="session token (hex)")
+    parser.add_argument("--identity-key", required=True, help="identity key (hex)")
+    parser.add_argument("--insecure", action="store_true", help="skip TLS verification")
+    args = parser.parse_args()
+
+    opts = ConnectOptions(
+        addr=args.server,
+        ca_cert_path=args.ca_cert,
+        server_name=args.server_name,
+        insecure_skip_verify=args.insecure,
+    )
+
+    token = bytes.fromhex(args.token)
+    identity_key = bytes.fromhex(args.identity_key)
+
+    asyncio.run(run_bot(opts, token, identity_key))
+
+
+if __name__ == "__main__":
+    main()