feat(asn): IP→ASN enrichment via iptoasn.com bulk dump

Mirrors decnet/geoip/ end-to-end: paths/base/factory/lookup at the
package level, iptoasn/ subpackage holds the data-source-specific
fetch+parse+provider. AsnLookup is bisect-indexed over (start, end,
AsnInfo) ranges with a pickled cache invalidated on raw-file mtime
bump.

Why iptoasn (and not bgp.tools / Team Cymru): public-domain dump,
zero attribution, no UA mandate, daily refresh — keeps DECNET stealth
intact (the geoip/rir module's "never identify as DECNET" comment
applies the same way here). bgp.tools' ToS would have required an
identifying UA, conflicting with feedback_stealth.

Public surface: decnet.asn.enrich_ip(ip) -> (asn, name, source) or
all-None on miss/disabled. Same shape as decnet.geoip.enrich_ip so
the profiler can compose them in one call site.

decnet/asn/iptoasn/__init__.py

@@ -0,0 +1,9 @@
+"""iptoasn.com IP→ASN provider.
+
+Daily-refreshed gzipped TSV dump of the global BGP table, derived from
+RIPE RIS. Released into the public domain by upstream — no attribution
+required, no UA mandate, no terms to violate.
+
+Direct imports of :class:`IptoasnProvider` are discouraged — go through
+:func:`decnet.asn.factory.get_provider`.
+"""