feat(ppi): add S7-200 serial transport

[gijzelaerr]

Summary

• add SD1, SD2, SD3, and short-confirmation PPI frame codecs
• add an optional pyserial transport implementing the S7-200 request, acknowledgement, poll, and response exchange
• add a PPI client with S7 PDU negotiation and read/write support for S, SM, AI, AQ, I, Q, M, V, counter, and timer areas
• document installation and usage, and cover framing, retries, area mappings, and client behavior with tests

Initial scope

This is the first point-to-point PC-master/single-slave milestone for #614. Multi-master token passing and PPI-over-TCP are intentionally deferred until protocol traces or suitable hardware are available. The implementation is marked experimental and still needs validation against a real S7-200 PLC.

The wire exchange and area mappings follow the mature libnodave implementation:
https://github.com/netdata-be/libnodave/blob/master/nodave.c#L4077-L4220

Validation

• uv run --default-index https://pypi.org/simple --frozen pre-commit run --all-files
• uv run --default-index https://pypi.org/simple --frozen pytest -q (1665 passed, 82 skipped)
• uv build --default-index https://pypi.org/simple