Add Inverter Power switch for NEO inverters

[aseemann]

Summary

Adds an Inverter Power switch entity to the NEO holding-register map. Writes register 0 with 1 = ON, 0 = OFF — the same command Growatt's cloud sends when the user taps the on/off button in ShinePhone.

This complements the existing output_power_limit (register 3), which can't reach a true off — the firmware enforces a ~30 W floor at 0 %.

Why

Users want a hard "disable inverter" trigger from Home Assistant automations (e.g. negative grid pricing, export caps). Register 3 alone can't do that; register 0 can.

How it was verified

1. Captured the cloud→inverter traffic on s/33/<serial> while toggling ShinePhone's on/off button. The downlink frame is a standard PRESET_SINGLE_REGISTER write (function 0x06) to register 0 with value 0x0000 (off) or 0x0001 (on). Frame layout matches GroBro's existing GrowattModbusFunctionSingle.build_grobro output byte-for-byte.
2. Built a standalone publisher that synthesizes the same frame from scratch; confirmed identical bytes against the capture.
3. Verified the inverter accepts the command:
   • while logged in to ShinePhone ✓
   • while logged out of ShinePhone ✓
   • via direct MQTT publish on the local broker (no Growatt cloud involvement) ✓
4. Verified that the HA discovery already wires type: switch end-to-end (precedent: NEXA Grid Power Allowed and the slot-enabled switches in growatt_nexa_registers.json). No Python changes required.

Tested with a Growatt NEO 800M-X (datalogger serial prefix QMN).

Notes / open follow-ups

• After a write to register 0, GroBro's existing read-after-write path (ha/client.py:428-430) issues a READ_SINGLE_REGISTER on register 0. I haven't confirmed whether the inverter responds to that read; if it doesn't, the switch state may briefly show as unknown until the next manual toggle. The HA-side Inverter_Status sensor (StandbyStatus vs NormalStatus) reflects the real state regardless and can be used as an availability template if it matters.
• The change is data-only (one JSON entry + a CHANGELOG.md note + a CONFIGURATION.md note in the Hardware Safety section). No code changes, no tests touched.

Test plan

• Pull the branch, restart GroBro.
• In HA, see a new switch.<serial>_inverter_power entity.
• Toggle off → inverter Pac drops to 0 within ~10 s.
• Toggle on → inverter resumes producing.
• (Maintainer) verify the EEPROM-wear note in CONFIGURATION.md is worded appropriately for register 0 — happy to adjust if you have better info.

[robertzaage]

@Lisugu00, sorry for bothering you again, but do you know if writing to holding register 0 causes any EEPROM wear?

[robertzaage]

@aseemann Thanks for your PR! 😊 Let's wait for a confirmation, if using this register in automations is EEPROM safe.