Add core traversal engine with iterative resolution from root servers:
- referral.go: Referral struct with name, type, bailiwick, addresses, state
- cache.go: Chained InfoCache with parent inheritance for NS and glue records
- stack.go: LIFO stack with configurable max depth enforcement
- response.go: Response classifier (referral, answer, CNAME follow, NODATA,
NXDOMAIN, SERVFAIL) with child referral generation and probability splitting
- traverser.go: Traverser orchestrates full traversal from root to leaf,
following all referral branches, handling CNAME chains, and respecting
max depth limits
Add IterativeQuery/IterativeQueryWithExchange to dns package (RD=false queries).
57 new tests covering all acceptance criteria: traversal from root to leaf,
comprehensive branch following, max depth enforcement, probability distribution,
chained cache behavior, CNAME following, NXDOMAIN/SERVFAIL handling, context
cancellation, and concurrent access.
Co-authored-by: multica-agent <github@multica.ai>
