Split monolithic __init__.py into six modules matching the Rust client
structure: protocol.py, codec.py, discovery.py, transport.py, connect.py,
client.py. All symbols re-exported from top-level for backwards compat.

New features:
- Ping-on-connect validation (200ms handshake, matches Rust v0.2.0)
- try_connect(), connect_default(), connect_mux_auto(), validate()
- connect() commits to mux when socket exists (no USB fall-through)
- Bandwidth/ErrorCode IntEnums, RadioConfig TypedDict, py.typed marker
- MAX_PAYLOAD, RADIO_CONFIG_SIZE, tag/error constants
- Payload size validation on Transmit
- 46 pytest tests ported from Rust (protocol, codec, client, firmware compat)
- GitHub Actions CI (ruff + pytest on 3.10-3.13 + PyPI publish)
- Strict ruff linting (E, W, F, I, UP, B, C4, ARG, SIM, RUF)

Breaking: package renamed back to donglora (from donglora-python).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>