Closes the NotSupportedException cliff for S7 Float64/Int64/UInt64.
- S7Size enum gains LWord (8 bytes); parser accepts DBLD/DBL on data
blocks and LD on M/I/Q (e.g. DB1.DBLD0, DB1.DBL8, MLD0, ILD8, QLD16).
- S7Driver.ReadOneAsync / WriteOneAsync issue ReadBytesAsync /
WriteBytesAsync for 64-bit types and convert big-endian via
System.Buffers.Binary.BinaryPrimitives. S7's wire format is BE.
- Internal MapArea(S7Area) helper translates to S7.Net DataType.
- MapDataType now surfaces native DriverDataType for Int16/UInt16/
UInt32/Int64/UInt64 instead of collapsing them all to Int32.
Tests: parser theories cover DBLD/DBL/MLD/ILD/QLD; discovery test
asserts the 64-bit DriverDataType mapping. 64/64 passing.
Closes#287
