Expose additional ASN fields from iptoasn lookup

[steffenfritz]

Summary

• Displays four additional fields from the existing iptoasn BGP lookup: AS Name, BGP Prefix, AS Registry, and AS Allocated date
• No new dependency — all fields are already returned by github.com/jamesog/iptoasn but were previously unused
• TSV output updated to include the new fields

Example output (new fields)

AS Number    : 3320
AS Name      : DTAG, DE
AS Country   : DE
AS Registry  : ripencc
AS Allocated : 1997-03-17
BGP Prefix   : 85.214.0.0/15

Test plan

• Run mxcheck -s <domain> -n and verify the new ASN fields appear in the output
• Run mxcheck -s <domain> -n -w and verify the new fields appear in the TSV report
• go vet ./... and go test -v ./... — all green

Closes #23

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