Add MicroPython/CircuitPython compatibility layer

[gijzelaerr]

Summary

• Add snap7/compat.py with runtime detection (MICROPYTHON flag), a functools.cache fallback for MicroPython, and a HAS_CTYPES availability flag
• Update snap7/error.py to import cache from the new compat module instead of functools directly
• Add tests/test_micropython_compat.py with 29 tests verifying that core protocol modules work using only struct/bytearray operations (no ctypes dependency)

Compatibility audit results

Module	Status	Notes
connection.py	OK	socket, struct, enum, logging -- all in MicroPython
s7protocol.py	OK	struct, enum, logging, datetime
datatypes.py	OK	struct, enum only
error.py	Shimmed	functools.cache replaced with compat shim
client.py	Partial	Core read/write works; async helpers need ctypes
type.py	Blocked	Heavy ctypes.Structure usage (BlocksList, S7CpuInfo, etc.)
util/getters.py	OK	struct, datetime only
util/setters.py	OK	struct, datetime, re only
util/db.py	OK	Pure Python data handling

What works on MicroPython today

The core protocol stack (connection.py, s7protocol.py, datatypes.py) and the utility getters/setters can run on MicroPython as-is. The snap7/compat.py module provides the MICROPYTHON flag and shims needed by error.py.

Known blockers for full MicroPython support

1. type.py uses ctypes.Structure extensively -- needs pure-Python dataclass replacements for MicroPython
2. client.py async helpers use ctypes.memmove and ctypes.Array -- core sync operations work fine
3. importlib.metadata in __init__.py -- not available in MicroPython

Test plan

• All 29 new compat tests pass
• All 476 existing tests pass (48 skipped -- need PLC)
• mypy clean (Success: no issues found in 29 source files)
• ruff clean (All checks passed!)

Closes #624

🤖 Generated with Claude Code