Implement MCNS v1: custom Go DNS server replacing CoreDNS

Replace the CoreDNS precursor with a purpose-built authoritative DNS
server. Zones and records (A, AAAA, CNAME) are stored in SQLite and
managed via synchronized gRPC + REST APIs authenticated through MCIAS.
Non-authoritative queries are forwarded to upstream resolvers with
in-memory caching.

Key components:
- DNS server (miekg/dns) with authoritative zone handling and forwarding
- gRPC + REST management APIs with MCIAS auth (mcdsl integration)
- SQLite storage with CNAME exclusivity enforcement and auto SOA serials
- 30 tests covering database CRUD, DNS resolution, and caching

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

deploy/examples/mcns.toml

@@ -0,0 +1,21 @@
+[server]
+listen_addr = ":8443"
+grpc_addr   = ":9443"
+tls_cert    = "/srv/mcns/certs/cert.pem"
+tls_key     = "/srv/mcns/certs/key.pem"
+
+[database]
+path = "/srv/mcns/mcns.db"
+
+[dns]
+listen_addr = ":53"
+upstreams   = ["1.1.1.1:53", "8.8.8.8:53"]
+
+[mcias]
+server_url   = "https://svc.metacircular.net:8443"
+ca_cert      = ""
+service_name = "mcns"
+tags         = []
+
+[log]
+level = "info"