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/tests/test_wire.py

@@ -0,0 +1,41 @@
+"""Tests for the v2 wire format encoder/decoder."""
+
+from quicproquo.wire import (
+    HEADER_SIZE,
+    encode_frame,
+    decode_header,
+    HEALTH,
+    ENQUEUE,
+)
+
+
+def test_header_size():
+    assert HEADER_SIZE == 10
+
+
+def test_encode_decode_roundtrip():
+    payload = b"\x0a\x05hello"
+    frame = encode_frame(HEALTH, 42, payload)
+
+    assert len(frame) == HEADER_SIZE + len(payload)
+
+    method_id, req_id, length = decode_header(frame)
+    assert method_id == HEALTH
+    assert req_id == 42
+    assert length == len(payload)
+    assert frame[HEADER_SIZE:] == payload
+
+
+def test_empty_payload():
+    frame = encode_frame(ENQUEUE, 1, b"")
+    method_id, req_id, length = decode_header(frame)
+    assert method_id == ENQUEUE
+    assert req_id == 1
+    assert length == 0
+
+
+def test_decode_header_too_short():
+    import pytest
+
+    with pytest.raises(ValueError, match="header too short"):
+        decode_header(b"\x00\x01")