feat(core): expose constructors for public tagged values (#80)

## Summary

- expose constructor helpers for schema-backed DNS records, plan
operations, revocation states, resolver results, and Vercel account
contexts
- make the Promise DNS resolver result constructors available from the
runtime namespace
- keep `_tag` literals at protocol and decoding boundaries while
removing them from trusted producers

## Tests

- `bun run --filter domainkit typecheck`
- `bun run --filter domainkit test`
- `bun run --filter domainkit build`
- `bun run --filter domainkit test:artifact`