R07:L5 (Modbus half, former F2): ModbusTcpTransport builds the MBAP ADU in a
pooled buffer and reads the reply header into one per-transport buffer (sends are
single-flight under the gate). ModbusRtuOverTcpTransport writes its ADU into a
pooled buffer (ModbusRtuFraming.WriteAdu, byte-identical to BuildAdu), and
ReadResponsePduAsync reads and validates the whole frame in one pooled buffer
instead of five arrays. Only the returned PDU is allocated. The exchange stays one
async method: splitting it cost a state-machine box that ate the saving. Scratch
A/B per op: RTU deframe 352 -> 184 B, RTU send 640 -> 408 B, TCP send 864 -> 800 B;
timings within noise; Driver.Modbus.Tests 2 s before and after. NormalizeWordOrder
lives in ModbusDriver (not a transport) and is left alone.
Refs #747
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty