[Core/ZHA] Investigate sub-device support

[TheJulianJES]

Other integrations like Shelly in Home Assistant support sub-devices. So, a two-gang relay/switch is exposed as two sub devices.
This is something we should look into for ZHA.

Some outstanding questions:

• How many Zigbee devices use multiple endpoints when they should not?
• Can this be done automatically or are explicit quirks definitions needed?
• ...

[Hedda]

I believe this could also be very relevant to ESPHome-based Zigbee devices now that such are possible since ESPHome 2026.1.0

• https://esphome.io/changelog/2026.1.0/

  • https://esphome.io/changelog/2026.1.0/#zigbee-support-for-nrf52

    • https://esphome.io/components/zigbee/

ESPHome at least have native sub-device support for allow creating multiple sub-devices within a single ESP controller for logical entity grouping in Home Assistant:

• https://www.esphome.io/components/esphome/#sub-devices

ESPHome supports creating sub-devices within a single ESP controller. This allows you to group entities into logical devices that appear separately in Home Assistant. This is particularly useful when:

• One ESP acts as a hub/gateway for multiple physical devices (RF bridges, Modbus devices, etc.)
• A single ESP controls multiple zones or rooms
• You want better organization of entities in Home Assistant

[Hedda]

ESPHome at least have native sub-device support for allow creating multiple sub-devices within a single ESP controller:

• https://www.esphome.io/components/esphome/#sub-devices

Sub-device support was by the way initially added in 2025.7.0 and the release notes for that goes into some details how it function:

• https://esphome.io/changelog/2025.7.0/#sub-device-support

Sub-Device Support

Note

New Feature: Entities can now be organized into logical sub-devices, enabling better organization
and area management in Home Assistant.

ESPHome now supports sub-devices, a powerful new feature that allows you to logically group entities from a single
physical ESPHome device into multiple virtual devices within Home Assistant. This is particularly useful for complex
setups where one ESP32 controls sensors or actuators in different areas of your home.

Use cases and benefits:

• Multi-room sensors - One ESP32 with temperature sensors in different rooms can appear as separate devices per room

• Complex automation hubs - A single ESP32 controlling lights, fans, and sensors across multiple areas

• Better Home Assistant organization - Entities automatically appear in the correct areas and can be managed
  independently

• Cleaner dashboards - Related entities are grouped together logically rather than by physical device

How it works:

You can assign entities to sub-devices using the new device_id configuration option. First, define your devices
in the devices section under esphome:, then reference them in individual entities.

Example configuration:

esphome:
  name: multi_room_controller

  # Define areas (optional)
  areas:
    - id: living_room_area
      name: "Living Room"
    - id: bedroom_area
      name: "Bedroom"

  # Define devices
  devices:
    - id: living_room_device
      name: "Living Room Sensors"
      area_id: living_room_area
    - id: bedroom_device
      name: "Bedroom Sensors"
      area_id: bedroom_area

# Assign sensors to different devices
sensor:
  - platform: dht
    pin: GPIO4
    temperature:
      name: "Living Room Temperature"
      device_id: living_room_device
    humidity:
      name: "Living Room Humidity"
      device_id: living_room_device

  - platform: dht
    pin: GPIO5
    temperature:
      name: "Bedroom Temperature"
      device_id: bedroom_device
    humidity:
      name: "Bedroom Humidity"
      device_id: bedroom_device

In this example, Home Assistant will see two separate sensor devices (one for living room, one for bedroom)
even though they're controlled by a single ESPHome device.

This feature enhances the logical organization of your smart home while maintaining the efficiency of
centralized ESPHome device management.