The TTL computation and assignment loops treat OPT record's Hdr.Ttl
as a regular TTL, but per RFC 6891 it encodes EDNS0 metadata
(ExtRCode|Version|Flags). This corrupts cached responses causing
systemd-resolved to reject them with EDNS version 255.
Also fix pointer aliasing: storeCache() stored raw *dns.Msg pointer
so subsequent mutations by Exchange() corrupted cached data.
- Skip OPT records in all TTL loops (Exchange + loadResponse)
- Use message.Copy() in storeCache() to isolate cache from mutations
5a957fd750