- Add CHINT DDSU666 profile (ddsu666.yaml): FC03 holding registers, voltage/
current/active power (kW)/reactive power (kvar)/PF/frequency, import+export
active energy. Word/byte order left at big-endian as a documented best guess
(manual has no float example) — to be confirmed on-device.
- Add DDSU666-Modbus-Protocol.md reference extracted from the official manual,
plus the source PDF (parity with the SDM120 reference).
- Generalize driver.read_blocks to dispatch FC03 (holding) or FC04 (input)
based on a function_code argument (default 4, SDM120 behaviour unchanged);
the code is validated before any connection is attempted.
- Wire profile.function_code through the CLI read command, the background
poller, and the device /test endpoint — previously the profile field was
declared but never honoured (read path was hardcoded to FC04).
- Tests: default -> FC04, function_code=3 -> FC03 holding, invalid FC rejected
before connecting.
