feat: complete Phase 4.1 error handling and robustness (HAN-385)

- Fast mode cache isolation: TraverserConfig.Fast=false gives each
  referral its own independent InfoCache with no cross-branch glue
  inheritance; Fast=true (default) retains the shared root cache so
  earlier branch discoveries are reused
- Wire cfg.Fast from CLI config into TraverserConfig in main.go
- IDN/Punycode: NewReferral now converts unicode domain labels to their
  ACE/punycode form via golang.org/x/net/idna before querying, with a
  graceful fallback when conversion fails
- DNSSEC: hasFinalAnswer() now skips RRSIG records alongside CNAME so
  a signed referral does not prevent CNAME following
- Tests: DNSSEC RRSIG does not block CNAME follow, fast/non-fast cache
  isolation, 12-NS referral, IDN conversion, wildcard answer, long CNAME
  chain depth limit, partial branch failure with graceful degradation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: multica-agent <github@multica.ai>

go.sum

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 golang.org/x/sys v0.39.0/go.mod h1:OgkHotnGiDImocRcuBABYBEXf8A9a87e/uXjp9XT3ks=
+golang.org/x/text v0.32.0 h1:ZD01bjUt1FQ9WJ0ClOL5vxgxOI/sVCNgX1YtKwcY0mU=
+golang.org/x/text v0.32.0/go.mod h1:o/rUWzghvpD5TXrTIBuJU77MTaN0ljMWE47kxGJQ7jY=
 golang.org/x/tools v0.40.0 h1:yLkxfA+Qnul4cs9QA3KnlFu0lVmd8JJfoq+E41uSutA=
 golang.org/x/tools v0.40.0/go.mod h1:Ik/tzLRlbscWpqqMRjyWYDisX8bG13FrdXp3o4Sr9lc=