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/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