Adds PD (PID), MG (Message), PLS (Programmable Limit Switch) and BT
(Block Transfer) file types to the PCCC parser. New AbLegacyDataType
enum members (PidElement / MessageElement / PlsElement /
BlockTransferElement) plus per-type sub-element catalogue:
- PD: SP/PV/CV/KP/KI/KD/MAXS/MINS/DB/OUT as Float32; EN/DN/MO/PE/
AUTO/MAN/SP_VAL/SP_LL/SP_HL as Boolean (word-0 status bits).
- MG: RBE/MS/SIZE/LEN as Int32; EN/EW/ER/DN/ST/CO/NR/TO as Boolean.
- PLS: LEN as Int32 (bit table varies per PLC).
- BT: RLEN/DLEN as Int32; EN/ST/DN/ER/CO/EW/TO/NR as Boolean.
Per-family flags on AbLegacyPlcFamilyProfile gate availability:
- PD/MG: SLC500 + PLC-5 (operator + status bits both present).
- PLS/BT: PLC-5 only (chassis-IO block transfer is PLC-5-specific).
- MicroLogix + LogixPccc: rejected — no legacy file-letter form.
Status-bit indices match Rockwell DTAM / 1747-RM001 / 1785-6.5.12:
PD word 0 bits 0-8, MG/BT word 0 bits 8-15. PLC-set status bits
(PE/DN/SP_*; ST/DN/ER/CO/EW/NR/TO) are surfaced as ViewOnly via
IsPlcSetStatusBit, matching the Timer/Counter/Control pattern from
ablegacy-3.
LibplctagLegacyTagRuntime decodes PD non-bit members as Float32 and
MG/BT/PLS non-bit members as Int32; status bits route through GetBit
with the bit-position encoded by the driver via StatusBitIndex.
Tests: parser positive cases per family + negative cases per family,
catalogue + bit-index + read-only-bit assertions.
Closes#248
