feat(plc4net): add little-endian byte order support to the C# code generator (#2840)

* feat(plc4net): add little-endian byte order support to ReadBuffer/WriteBuffer

Adds a byte-order stack (default BIG_ENDIAN) with PushByteOrder/PopByteOrder,
and wires it into the unsigned and signed multi-byte read/write paths. Single
bytes, bits, strings and byte arrays are unaffected - they have no byte order
to begin with. Little-endian float/double reads and writes fail loudly
(NotImplementedException) instead of silently computing a wrong value, since
no mspec here declares one yet.

The ByteOrder enum itself already existed (added in #2771) but had no
consumer anywhere in the codebase; this wires it up for the first time.

* feat(plc4net): emit the little-endian push/pop wrapper in the C# generator

ValidateModel now accepts byteOrder="LITTLE_ENDIAN" on a complex type; enums
and dataIo still require BIG_ENDIAN only, since neither has a parse/serialize
body to scope a push/pop to. A type that declares LITTLE_ENDIAN has its
generated StaticParse/Serialize push it before its own body and pop it in a
finally, mirroring the Java generator's pushContext/popContext - a nested
type that declares no byte order of its own inherits it from the same
buffer, because it parses or serializes while still inside the declaring
type's try.

Regenerating S7 and Modbus (previously the only two protocols that passed
ValidateModel) produces byte-for-byte unchanged output. The upstream SLMP
mspec, previously rejected outright for declaring LITTLE_ENDIAN, now
generates successfully end-to-end.