Closes the NotSupportedException cliff for S7 string-shaped types.
- S7DataType gains WString, Char, WChar members alongside the existing
String entry.
- New S7StringCodec encodes/decodes the four wire formats:
STRING : 2-byte header (max-len + actual-len bytes) + N ASCII bytes
-> total 2 + max_len.
WSTRING : 4-byte header (max-len + actual-len UInt16 BE) + N×2
UTF-16BE bytes -> total 4 + 2 × max_len.
CHAR : 1 ASCII byte (rejects non-ASCII on encode).
WCHAR : 2 UTF-16BE bytes.
Header-bug clamp: actualLen > maxLen is silently clamped on read so
firmware quirks don't walk past the wire buffer; rejected on write
to avoid silent truncation.
- S7Driver.ReadOneAsync / WriteOneAsync issue ReadBytesAsync /
WriteBytesAsync against the parsed Area / DbNumber / ByteOffset and
honour S7TagDefinition.StringLength (default 254 = S7 STRING max).
- MapDataType returns DriverDataType.String for the three new enum
members so OPC UA discovery surfaces them as scalar strings.
Tests: 21 new cases on S7StringCodec covering golden-byte vectors,
encode/decode round-trips, the firmware-bug header-clamp, ASCII-only
guard on CHAR, and the StringLength default. 85/85 passing.
Closes#288
