Build: Ship the DNS Server — Milestones 4–5

Your server answers hardcoded A records. This module's material — binary protocols, UDP, concurrency at the socket level — finishes it.

Milestone 4 — Real zone, real answers

1. Zone file loader: parse the zones.conf format from the project page into map[string][]Record at startup.
2. NXDOMAIN: RCODE 3 for domains not in the zone.
3. All five record types: A, AAAA, CNAME, MX, TXT — each has its RDATA format in the project page's table.
4. Name compression: handle pointer bytes (0xC0 ...) when decoding, use 0xC00C when encoding answers.

Done when every dig example in the project page's Usage section returns the right records, and dig nope.local returns status NXDOMAIN.

Milestone 5 — Concurrent + forwarding

1. One goroutine per query. Prove it holds up:

   for i in $(seq 50); do dig @localhost -p 5353 web.local A & done; wait
   

   Fifty clean answers, and go test -race ./... stays green.

2. Upstream forwarding: --upstream 8.8.8.8:53 relays unknown domains and returns the upstream answer.

3. README with a real dig session transcript. Push it.

Shipped: three of five. "I built a DNS server in Go from scratch" — that sentence is now true, and you have the repo to prove it.