Group writes by device through new AbCipMultiWritePlanner; for families that
support CIP request packing (ControlLogix / CompactLogix / GuardLogix) the
packable writes for one device are dispatched concurrently so libplctag's
native scheduler can coalesce them onto one Multi-Service Packet (0x0A).
Micro800 keeps SupportsRequestPacking=false and falls back to per-tag
sequential writes. BOOL-within-DINT writes are excluded from packing and
continue to go through the per-parent RMW semaphore so two concurrent bit
writes against the same DINT cannot lose one another's update.
The libplctag .NET wrapper does not expose a Multi-Service Packet construction
API at the per-Tag surface (each Tag is one CIP service), so this PR uses
client-side coalescing — concurrent Task.WhenAll dispatch per device — rather
than building raw CIP frames. The native libplctag scheduler does pack
concurrent same-connection writes when the family allows it, which gives the
round-trip reduction #228 calls for without ballooning the diff.
Per-tag StatusCodes preserve caller order across success, transport failure,
non-writable tags, unknown references, and unknown devices, including in
mixed concurrent batches.
Closes#228
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.
