feat(modbus): add DDSU666 profile and select read function code per profile

- 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.

app/integrations/modbus/driver.py

@@ -1,8 +1,11 @@
 """Modbus TCP driver — thin wrapper around pymodbus.
 
 This module provides a single public function ``read_blocks`` that performs
-one or more FC04 (Read Input Registers) block reads against a Modbus TCP
-gateway and returns a flat ``dict[register_address -> 16-bit_value]`` map.
+one or more block reads against a Modbus TCP gateway — using either FC04
+(Read Input Registers) or FC03 (Read Holding Registers), selected per call
+via the ``function_code`` argument — and returns a flat
+``dict[register_address -> 16-bit_value]`` map.  The function code comes from
+the device profile (e.g. SDM120 uses FC04, DDSU666 uses FC03).
 
 Design decisions
 ----------------
@@ -80,9 +83,10 @@ def read_blocks(
     unit_id: int,
     blocks: Sequence[Block],
     *,
+    function_code: int = 4,
     timeout: float = 3.0,
 ) -> dict[int, int]:
-    """Read one or more contiguous register blocks via FC04 (input registers).
+    """Read one or more contiguous register blocks via FC03 or FC04.
 
     Parameters
     ----------
@@ -96,6 +100,11 @@ def read_blocks(
         Sequence of ``{"start": int, "count": int}`` dicts describing the
         contiguous register ranges to read.  ``count`` is the number of
         16-bit registers (not bytes).
+    function_code:
+        Modbus read function code: ``4`` for FC04 (Read Input Registers,
+        default — SDM120) or ``3`` for FC03 (Read Holding Registers —
+        DDSU666 and other devices that expose measurements as holding
+        registers).  Comes from the device profile's ``function_code`` field.
     timeout:
         TCP connect/read timeout in seconds (default 3 s).
 
@@ -107,12 +116,21 @@ def read_blocks(
 
     Raises
     ------
+    ModbusDriverError
+        If ``function_code`` is neither 3 nor 4 (validated before any
+        connection is attempted).
     ModbusConnectionError
         If the TCP connection to the gateway fails.
     ModbusResponseError
         If the gateway returns a Modbus exception frame or an unexpected
         number of registers.
     """
+    if function_code not in (3, 4):
+        raise ModbusDriverError(
+            f"Unsupported read function code FC{function_code:02d} "
+            f"(only FC03 holding-register and FC04 input-register reads are supported)"
+        )
+
     client = ModbusTcpClient(host, port=port, timeout=timeout)
     try:
         connected = client.connect()
@@ -125,7 +143,7 @@ def read_blocks(
         for block in blocks:
             start: int = block["start"]
             count: int = block["count"]
-            _read_block(client, unit_id, start, count, registers)
+            _read_block(client, unit_id, start, count, registers, function_code=function_code)
 
         return registers
 
@@ -145,10 +163,19 @@ def _read_block(
     start: int,
     count: int,
     result: dict[int, int],
+    *,
+    function_code: int,
 ) -> None:
-    """Read one block and merge into *result*.  Raises on any error."""
+    """Read one block and merge into *result*.  Raises on any error.
+
+    ``function_code`` is assumed already validated to be 3 or 4 by the caller
+    (``read_blocks``); 3 dispatches FC03 (holding) and 4 dispatches FC04 (input).
+    """
     try:
-        response = client.read_input_registers(start, count=count, device_id=unit_id)
+        if function_code == 3:
+            response = client.read_holding_registers(start, count=count, device_id=unit_id)
+        else:  # function_code == 4 (input registers)
+            response = client.read_input_registers(start, count=count, device_id=unit_id)
     except ConnectionException as exc:
         raise ModbusConnectionError(
             f"Lost connection while reading registers 0x{start:04X}+{count}: {exc}"

app/integrations/modbus/profiles/ddsu666.yaml

@@ -0,0 +1,82 @@
+name: ddsu666
+description: CHINT DDSU666 single-phase smart meter
+function_code: 3          # holding registers (FC03) — DDSU666 has NO input registers (no FC04)
+word_order: big           # high register first — ASSUMED; verify with a known voltage reading
+byte_order: big           # high byte first within each register (confirmed by manual CRC example)
+
+# NOTE: the manual gives no worked float-decode example, so word_order is a best-guess
+# (standard big-endian, high register first, matching sdm120).  After wiring the meter,
+# read 0x2000 (voltage) — it should decode to ~230 V.  If it decodes to garbage, the
+# device uses the opposite word order and this profile (and the decoder) need adjusting.
+# Byte order is confirmed big-endian from the manual (Appendix A, Table A.4: 0x1388 -> 13 88).
+
+blocks:
+  # Instantaneous quantities 0x2000–0x200F: voltage, current, P, Q, (rsv), PF, (rsv), Freq.
+  # 16 contiguous registers — single bulk read.  (DDSU666 manual Table 9.)
+  - { start: 0x2000, count: 0x0010 }
+  # Active energy — import (0x4000) and export (0x400A) read as two small blocks rather
+  # than one span, to avoid touching the undocumented/reserved 0x4002–0x4009 gap.
+  - { start: 0x4000, count: 0x0002 }
+  - { start: 0x400A, count: 0x0002 }
+
+metrics:
+  # Addresses are the raw Modbus protocol addresses (hex) from DDSU666 manual Table 9,
+  # read via FC03.  Each float32 occupies two consecutive 16-bit registers.
+
+  - key: voltage
+    address: 0x2000        # U — Voltage (V)
+    type: float32
+    unit: "V"
+    device_class: voltage
+    ha_component: sensor
+
+  - key: current
+    address: 0x2002        # I — Current (A)
+    type: float32
+    unit: "A"
+    device_class: current
+    ha_component: sensor
+
+  - key: active_power
+    address: 0x2004        # P — Active power.  Manual unit is kW (NOT W like sdm120).
+    type: float32
+    unit: "kW"
+    device_class: power
+    ha_component: sensor
+
+  - key: reactive_power
+    address: 0x2006        # Q — Reactive power (kvar)
+    type: float32
+    unit: "kvar"
+    device_class: reactive_power
+    ha_component: sensor
+
+  - key: power_factor
+    address: 0x200A        # PF — Power factor (dimensionless)
+    type: float32
+    unit: ""
+    device_class: power_factor
+    ha_component: sensor
+
+  - key: frequency
+    address: 0x200E        # Freq — Frequency (Hz)
+    type: float32
+    unit: "Hz"
+    device_class: frequency
+    ha_component: sensor
+
+  - key: import_energy
+    address: 0x4000        # Ep — positive/forward active energy (kWh)
+    type: float32
+    unit: "kWh"
+    device_class: energy
+    state_class: total_increasing
+    ha_component: sensor
+
+  - key: export_energy
+    address: 0x400A        # -Ep — reverse active energy (kWh)
+    type: float32
+    unit: "kWh"
+    device_class: energy
+    state_class: total_increasing
+    ha_component: sensor