Add resolver API definition and comprehensive test suite

35 tests define the full resolver contract using live DNS queries
against *.dns.sneak.cloud (Cloudflare). Tests cover:
- FindAuthoritativeNameservers: iterative NS discovery, sorting,
  determinism, trailing dot handling, TLD and subdomain cases
- QueryNameserver: A, AAAA, CNAME, MX, TXT, NXDOMAIN, per-NS
  response model with status field, sorted record values
- QueryAllNameservers: independent per-NS queries, consistency
  verification, NXDOMAIN from all NS
- LookupNS: NS record lookup matching FindAuthoritative
- ResolveIPAddresses: basic, multi-A, IPv6, dual-stack, CNAME
  following, deduplication, sorting, NXDOMAIN returns empty
- Context cancellation for all methods
- Iterative resolution proof (resolves example.com from root)

Also adds DNSSEC validation to planned future features in README.

go.mod

@@ -9,6 +9,7 @@ require (
 	github.com/joho/godotenv v1.5.1
 	github.com/prometheus/client_golang v1.23.2
 	github.com/spf13/viper v1.21.0
+	github.com/stretchr/testify v1.11.1
 	go.uber.org/fx v1.24.0
 	golang.org/x/net v0.50.0
 )
@@ -16,10 +17,12 @@ require (
 require (
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/prometheus/client_model v0.6.2 // indirect
 	github.com/prometheus/common v0.66.1 // indirect
 	github.com/prometheus/procfs v0.16.1 // indirect
@@ -37,4 +40,5 @@ require (
 	golang.org/x/sys v0.41.0 // indirect
 	golang.org/x/text v0.34.0 // indirect
 	google.golang.org/protobuf v1.36.8 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 )