feat: Add MAC address to device registry via ARP discovery

[Splint77]

Is your feature request related to a problem? Please describe.

No response

Describe the solution you'd like

Feature request

I would like to propose adding MAC address discovery to the device registry entry for the Intex Spa.

What

When the integration sets up a config entry, it attempts to resolve the spa's IP address to a MAC address by reading the system ARP table (/proc/net/arp). If a valid MAC is found, it is stored in entry.data and registered as a CONNECTION_NETWORK_MAC connection in DeviceInfo.

This allows Home Assistant to:

• Uniquely identify the device by its MAC address (not just the integration UID)
• Improve device deduplication across restarts or IP changes

Why

Currently the device is only identified by the domain + UID. Adding a MAC address connection makes the device more robustly identifiable in the HA device registry.

Implementation

I have already implemented this in a dedicated branch:
https://github.com/Splint77/homeassistant-intex-spa/tree/feat/mac-address

Changes are minimal:

• __init__.py: ARP lookup function + stores MAC in entry.data on first setup
• entity.py: Adds CONNECTION_NETWORK_MAC to DeviceInfo if MAC is available

Happy to open a pull request if this is of interest!

Describe alternatives you've considered

n/a

Additional context

No response

Checklist

• I have filled out the template to the best of my ability.
• This only contains 1 feature request (if you have multiple feature requests, open one feature request for each feature request).
• This issue is not a duplicate feature request of previous feature requests.