feat: build warns when a lexicon's DNS record is missing #15

build now looks up _lexicon.<authority> for each lexicon it's building, wherever the file lives, and warns with the exact TXT record to create if it doesn't name the site's DID. It's a warning rather than a failure because people sometimes publish before they've set the DNS up (and might well use this to find out which record they need); check still fails on it after deploy. A lookup that fails outright (eg. when offline) says it couldn't verify instead, so nobody's told to create a record they may already have. The lookup lived in check, so it's moved to records.CheckLexiconDNS for both to share.