Closes#226
Adds nullable StringLength to AbCipTagDefinition + AbCipStructureMember
so STRING_20 / STRING_40 / STRING_80 UDT variants decode against the
right DATA-array capacity. The configured length threads through a new
StringMaxCapacity field on AbCipTagCreateParams and lands on the
libplctag Tag.StringMaxCapacity attribute (verified property on
libplctag 1.5.2). Null leaves libplctag's default 82-byte STRING in
place for back-compat. Driver gates on DataType == String so a stray
StringLength on a DINT tag doesn't reshape that buffer. UDT member
fan-out copies StringLength from the AbCipStructureMember onto the
synthesised member tag definition.
Tests: 4 new in AbCipDriverReadTests covering threaded StringMaxCapacity,
the null back-compat path, the non-String gate, and the UDT-member fan-out.
