docs(sdk): add comprehensive SDK documentation and wire format reference

Covers all official SDKs (Rust, Go, Python, TypeScript, C FFI),
the v2 wire format with method ID tables, authentication flow,
and a build-your-own-SDK guide with implementation checklist.

docs/sdk/python.md

@@ -0,0 +1,120 @@
+# Python SDK
+
+The Python SDK provides both async QUIC transport and synchronous Rust FFI backends.
+
+Location: `sdks/python/`
+
+## Installation
+
+```sh
+pip install quicproquo
+```
+
+## Transport Backends
+
+### Async QUIC (pure Python)
+
+Uses [aioquic](https://github.com/aiortc/aioquic) for native QUIC transport with the v2 protobuf wire format. No Rust build required.
+
+```python
+import asyncio
+from quicproquo import QpqClient, ConnectOptions
+
+async def main():
+    client = await QpqClient.connect(ConnectOptions(
+        addr="127.0.0.1:5001",
+        ca_cert_path="ca.pem",
+    ))
+
+    health = await client.health()
+    print(f"Server: {health.status}")
+
+    # OPAQUE auth (requires external OPAQUE library)
+    resp = await client.login_start("alice", opaque_request)
+    token = await client.login_finish("alice", finalization, identity_key)
+
+    # Messaging
+    key, proof = await client.resolve_user("bob")
+    seq, proof = await client.send(key, b"hello")
+    messages = await client.receive_wait(my_key, timeout_ms=5000)
+
+    await client.close()
+
+asyncio.run(main())
+```
+
+### Rust FFI (synchronous)
+
+Wraps `libquicproquo_ffi` via CFFI for full Rust crypto stack at native speed.
+
+```sh
+cargo build --release -p quicproquo-ffi
+```
+
+```python
+from quicproquo import QpqClient, ConnectOptions
+
+client = QpqClient.connect_ffi(ConnectOptions(
+    addr="127.0.0.1:5001",
+    ca_cert_path="ca.pem",
+))
+
+client.ffi_login("alice", "password123")
+client.ffi_send("bob", b"hello")
+messages = client.ffi_receive(timeout_ms=5000)
+client.close_sync()
+```
+
+## Async API (QUIC)
+
+| Method | Description |
+|---|---|
+| `QpqClient.connect(opts)` | Connect via QUIC |
+| `client.health()` | Health check |
+| `client.register_start(username, request)` | OPAQUE register start |
+| `client.register_finish(username, upload, key)` | OPAQUE register finish |
+| `client.login_start(username, request)` | OPAQUE login start |
+| `client.login_finish(username, finalization, key)` | OPAQUE login finish |
+| `client.resolve_user(username)` | Look up identity key |
+| `client.resolve_identity(key)` | Reverse look up username |
+| `client.create_channel(peer_key)` | Create DM channel |
+| `client.send(recipient_key, payload)` | Send message |
+| `client.receive(recipient_key)` | Fetch messages |
+| `client.receive_wait(recipient_key, timeout_ms=5000)` | Long-poll |
+| `client.ack(recipient_key, seq_up_to)` | Acknowledge messages |
+| `client.upload_key_package(key, package)` | Upload MLS key package |
+| `client.fetch_key_package(key)` | Fetch MLS key package |
+| `client.upload_hybrid_key(key, hybrid_pk)` | Upload hybrid key |
+| `client.fetch_hybrid_key(key)` | Fetch hybrid key |
+| `client.delete_account()` | Delete account |
+| `client.close()` | Disconnect (async) |
+
+## FFI API (synchronous)
+
+| Method | Description |
+|---|---|
+| `QpqClient.connect_ffi(opts)` | Connect via Rust FFI |
+| `client.ffi_login(username, password)` | Full OPAQUE login |
+| `client.ffi_send(recipient, message)` | Send by username |
+| `client.ffi_receive(timeout_ms=5000)` | Receive messages |
+| `client.close_sync()` | Disconnect (sync) |
+
+## Exceptions
+
+| Exception | Description |
+|---|---|
+| `QpqError` | Base exception for all SDK errors |
+| `AuthError` | OPAQUE authentication failed |
+| `TimeoutError` | Operation timed out |
+| `ConnectionError` | Server connection failure |
+
+## Structure
+
+| File | Purpose |
+|---|---|
+| `quicproquo/client.py` | High-level client API |
+| `quicproquo/transport.py` | QUIC transport (aioquic) |
+| `quicproquo/ffi.py` | Rust FFI transport (CFFI) |
+| `quicproquo/proto.py` | Protobuf encode/decode |
+| `quicproquo/wire.py` | v2 wire format framing |
+| `quicproquo/types.py` | Data types and exceptions |