[Request]: DNSServer should support TCP transport (RFC 7766)

The bundled DNS server in `Sources/DNSServer/DNSServer.swift` is UDP-only today. Two TODO markers in the source flag this as known-incomplete:

- `DNSServer.swift:39` `// TODO: TCP server`
- `DNSServer.swift:62` `// TODO: TCP server`

RFC 7766 ("DNS Transport over TCP - Implementation Requirements") requires modern resolvers to support TCP both as a fallback when UDP responses exceed 512 bytes (TC bit set) and as a primary transport. Without TCP, these query classes return truncated data or fail inside containers:

- DNSSEC-signed responses (RRSIG records push past 512 bytes regularly)
- Large AAAA / SRV / TXT records
- SPF / DKIM TXT chains (`_spf.google.com` is a common offender)

This shows up in the wild as "DNS sometimes works" bugs that are hard to attribute to the bundled resolver.

Proposed scope: add a TCP listener bound to the same address as the UDP socket, parse RFC 1035 length-prefixed messages, dispatch to the existing `Handlers/` stack, write the response back with the length prefix. Reuses the parser and handlers - transport-layer change only. ~200 lines including tests.

Happy to put up a PR if this fits the project's direction. Filing the issue first since I didn't see one tracking it.

Start with Sources/DNSServer/DNSServer.swift and the two TODO markers at lines 39 and 62, then trace how the existing Handlers/ stack receives UDP messages. Add coverage for RFC 1035 length-prefixed TCP messages and verify that TCP responses use the same framing and handler path. Done means the listener shares the UDP address and supports oversized or TCP-primary DNS queries without changing existing UDP behavior.

Written by the indexing model from the issue text.