Move the owner-name rule into its own package

[vvoytovych]

Summary

The rule that decides whether two spellings of an owner name are one name lived inside the PowerDNS client, so the webhook and the status helpers imported the backend to use it, and the helpers kept a second copy. This moves the rule into a small package of its own, and the helpers now call it. Nothing changes in behaviour. It prepares the fix for #188, whose election of claimants must not depend on the backend.

Test plan

• Unit, envtest and PowerDNS integration tests pass, and lint is clean
• The status helpers give the same answer as before on every owner and zone pair tried, malformed ones included
• The CLI and the MCP server build and pass their tests

[0xmc]

@vvoytovych you state "Nothing changes in behaviour," but it looks like ownername now comes from milo instead of powerdns, which is an improvement. Is it because the owername strings are equal in all cases?

[vvoytovych]

@0xmc Yes, the strings are equal in every case, because the code is the same. go.miloapis.com/dns-operator is this repository's module path, not Milo. The owner name still comes from the record set's spec and its zone's domain, as before. Neither Milo nor PowerDNS is asked.

• ownername.Qualify is pdns.QualifyOwner moved as it was. The function body is byte for byte the same.
• The status helpers, which the CLI and the MCP server use for names people type, had a copy of their own. They now call ownername.Qualify when a zone is given, and they still trim spaces and accept a name without a zone. Against their old version they gave the same answer on 436,956 owner and zone pairs, malformed ones included, with no difference.

The reason for the move: code that compares owner names no longer has to import the PowerDNS client. The shared election function (step 3 of #201) needs that.