Add native AsyncClient with asyncio support

[gijzelaerr]

Summary

Adds a native AsyncClient for async S7 communication using asyncio streams. This replaces the previous asyncio.to_thread() approach (which was not safe for concurrent use) with proper non-blocking I/O, following the pattern from python-s7comm.

Key design decisions

• Native async I/O: Uses asyncio.open_connection() with StreamReader/StreamWriter instead of blocking sockets wrapped in threads. The event loop is never blocked.
• asyncio.Lock() in _send_receive(): S7 is request-response, so each send+receive cycle must be atomic. The lock serializes concurrent coroutines, making asyncio.gather() safe.
• Request-embedded sequence validation: The async client extracts the expected PDU sequence number directly from the request bytes (S7 header offset 4-5) rather than relying on the shared protocol counter, avoiding race conditions when multiple coroutines build requests concurrently.
• ClientMixin for shared logic: 14 pure-computation methods (parameter management, error text, area mapping, etc.) are shared between Client and AsyncClient via a mixin in snap7/client_base.py, eliminating code duplication.
• S7Protocol unchanged: The protocol layer is pure computation (struct pack/unpack) — no I/O, no async needed.

New files

• snap7/async_client.py — AsyncISOTCPConnection (async transport) and AsyncClient (full-featured async S7 client)
• snap7/client_base.py — ClientMixin with shared pure-computation methods
• tests/test_async_client.py — 26 tests covering connection, DB read/write, area read/write, concurrent safety, multi-var operations, and synchronous helpers

Feature parity

AsyncClient supports the same operations as the sync Client:

• connect / disconnect / get_connected
• db_read / db_write / db_get
• read_area / write_area (with chunked transfers for large payloads)
• ab_read / ab_write, eb_read / eb_write, mb_read / mb_write, tm_read / tm_write, ct_read / ct_write
• read_multi_vars / write_multi_vars
• list_blocks / list_blocks_of_type / get_block_info
• get_cpu_state / get_cpu_info / get_pdu_length
• upload / full_upload / download
• read_szl / get_plc_datetime / set_plc_datetime
• plc_hot_start / plc_cold_start / plc_stop
• get_cp_info / get_order_code / get_protection
• Parameter management, session passwords, connection params
• async with context manager

Test plan

• All 454 existing tests pass (428 sync + 26 new async)
• Concurrent safety validated: asyncio.gather() with multiple reads, mixed read/write, and 10 concurrent reads
• Ruff formatting checks pass

🤖 Generated with Claude Code