Implement Platform Specific ARP Table Lookups

**What are you trying to do?**
Prevent MAC address collisions by checking the host ARP/neighbor table before assigning a generated MAC address.

**What's your proposed solution?**
Extend `generate_unused_mac_address` to:

* Check the internal `VMSpecs` database (existing behavior)
* Query the host OS neighbor table (ARP cache)
* Regenerate the MAC if a conflict is found

Platform-specific approach:

* **macOS:** Use `sysctl` with `NET_RT_FLAGS` and `RTF_LLINFO` to inspect ARP entries
* **Windows:** Use `GetIpNetTable2` from `IPHLPAPI.lib` to query the neighbor cache

**Additional context**

As of now, MAC validation is limited to Multipass-managed instances.
This can likely cause collisions with external devices on the same LAN.

Start by locating generate_unused_mac_address and reviewing its existing VMSpecs database check. Then investigate the proposed macOS sysctl and Windows GetIpNetTable2 neighbor-table entry points. Done means generated MAC addresses are checked against host neighbor data and regenerated on conflicts, while preserving the existing validation.

Written by the indexing model from the issue text.