Add Resolver interface, BasicResolver, and CachingResolver to internal/dns.
CachingResolver wraps any Resolver with an in-memory cache keyed by
(server IP, name, type, class). Cache entries respect DNS TTL from
responses, falling back to a configurable default TTL. Thread-safe
using sync.RWMutex. Includes Len, Clear, and PurgeExpired methods
for cache management.
Closes HAN-379 (Phase 2.2).
Co-authored-by: multica-agent <github@multica.ai>
