fix: merge address info when aliasing entries to handle out-of-order DNS records

[Yanhu007]

Fixes #145

Problem

When DNS records arrive in certain orders (A/AAAA before SRV), the alias function overwrites an existing entry that already has an IP address with the SRV entry that only has port/host info. The IP address is lost, and the entry never becomes complete.

Example failure sequence:

1. PTR → creates entry
2. A record for device.local. → creates entry with IP
3. SRV for device._service._tcp.local. with target device.local. → alias() overwrites the A-record entry, losing the IP
4. Entry never completes because it has port but no IP

This was reported with Alfen EV Chargers but affects any device that sends A/AAAA records before SRV records.

Fix

In alias(), when the destination key already has an entry with address information, merge the addresses into the source entry before overwriting the map entry.

Testing

All existing tests pass.

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[hashicorp-cla-app]

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