fix: track modified_instances when removing an interface

[keepsimple1]

Fixes: #447

Problem

When a network interface was removed, remove_records_on_intf only tracked instances whose PTR records were fully gone (i.e. no remaining PTR on any interface). Instances that still had a PTR record but lost address or other records on the removed interface were silently ignored — their cached ServiceInfo became stale without any notification to listeners.

Changes

• Updated remove_records_on_intf to return both removed_instances and modified_instances.
• Added unit test covering the case where an ADDR record is removed on one interface.
• Updated the call site to call rresolve_updated_instances for modified_instances after interface removal, triggering a new ServiceResolved event with the updated (reduced) address set.