feat(dns): record per-query inspection timing

[ryanfowler]

Summary

• record elapsed time for each concurrently issued DNS inspection query
• keep shared resolver setup out of per-query timings
• verify direct DoH query results expose positive timing metadata

Validation

• go test -v -parallel 8 ./...
• go vet ./...
• go test -race ./internal/dnsinspect
• staticcheck ./...
• go build .
• go mod verify