Adds ModbusDriverOptions knobs that ≥4 of 6 surveyed vendors expose:
1. MaxCoilsPerRead (ushort, default 2000) — separate from MaxRegistersPerRead
because coil packing (1 bit per coil) and register packing (16 bits each)
have different spec ceilings. Coil-array reads above the cap auto-chunk
the same way register reads have always done. New ReadBitBlockChunkedAsync
re-assembles per-chunk LSB-first bitmaps into one logical bitmap.
2. UseFC15ForSingleCoilWrites (default false) — forces FC15 (Write Multiple
Coils with quantity=1) for single-coil writes instead of the default FC05
(Write Single Coil). Safety / audit PLCs that only accept the multi-write
codes need this.
3. UseFC16ForSingleRegisterWrites (default false) — same idea for FC16 vs
FC06 on single holding-register writes.
4. DisableFC23 (default false) — placeholder no-op for the future block-read
coalescing (#143) work that may opt into FC23 (Read/Write Multiple
Registers). Lets deployments pre-disable FC23 for PLCs that won't accept
it, before we ship the optimisation that emits it.
Defaults preserve the historical wire output bit-for-bit (FC05/FC06 for
singles, no chunking under 2000 coils, no FC23). Factory DTO + JSON-binding
extended with parallel fields.
6 new ModbusProtocolOptionsTests covering: defaults, FC05→FC15 forcing,
FC06→FC16 forcing, MaxCoilsPerRead chunking math (2500 coils / 2000 cap →
2 reads of 2000 + 500). Existing 209 unit tests still green.
55f4044a69