Add dual-dialect Tag parsing (PLC4X + nodeS7)

[gijzelaerr]

Summary

• PLC4XTag and NodeS7Tag are new subtypes of Tag, each with their own .parse() classmethod and a __str__ that round-trips to the source dialect.
• parse_tag(s, *, strict=True) is a dispatcher that autodetects dialect from syntax markers: , selects nodeS7, :TYPE selects PLC4X. In strict mode, bare short forms like M7.1 or IW22 raise; strict=False dispatches them to the nodeS7 parser (which accepts bareness).
• Tag.from_string stays as an alias for PLC4XTag.parse for backwards compatibility. All existing callers and tests continue to work.

Why nodeS7?

pyS7 and the Node-RED ecosystem use DB1,X0.0 / DB1,R4 / M10.5 / IW22. Accepting that syntax alongside PLC4X makes it painless for existing users of those tools (including the competing ha-s7plc HA integration) to migrate to python-snap7 4.0. The canonical in-memory representation is unchanged; only the edges (parse + __str__) know about dialect.

New API

from snap7 import PLC4XTag, NodeS7Tag, parse_tag

PLC4XTag.parse("DB1.DBD0:REAL")        # strict PLC4X
NodeS7Tag.parse("DB1,R0")              # strict nodeS7

parse_tag("DB1.DBD0:REAL")             # → PLC4XTag
parse_tag("DB1,R0")                    # → NodeS7Tag
parse_tag("M7.1")                      # raises (ambiguous)
parse_tag("M7.1", strict=False)        # → NodeS7Tag (BOOL, bit 1 of byte 7)
parse_tag("IW22", strict=False)        # → NodeS7Tag (WORD)

str(PLC4XTag.parse("DB1,DBX0.0"))      # not valid; pass :BOOL
str(NodeS7Tag.parse("DB1,R4"))         # "DB1,R4" (round-trip)

Test plan

• 91 tag tests pass (new + existing)
• Full suite: 1497 passed, 91 skipped
• mypy clean across snap7/s7/tests/example
• ruff check and format clean
• pre-commit all hooks pass

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