Fix Lookup instance filtering

[HarshDevelops]

Closes #120

What was wrong

Resolver.Lookup accepted an instance name, but the internal full service
name was created before that instance was added. Because of that, responses
from other service instances could also be returned.

What changed

Lookup parameters are now created with the instance name from the start, so
responses are filtered to the requested service instance.

The default local domain behaviour remains unchanged.

Testing

• Added a regression test for the requested instance name
• Verified the previous construction path left the full instance name empty
• go test ./... could not complete locally because the installed Go 1.26.4
  toolchain fails linking the repository's older golang.org/x/net dependency:
  invalid reference to syscall.recvmsg