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.

tests/asn/test_parse.py

@@ -0,0 +1,57 @@
+"""Parser tests for the iptoasn TSV dump."""
+from __future__ import annotations
+
+import gzip
+import ipaddress
+from pathlib import Path
+
+from decnet.asn.iptoasn.parse import parse_file
+
+
+_FIXTURE_TSV = (
+    "1.0.0.0\t1.0.0.255\t13335\tUS\tCLOUDFLARENET\n"
+    "8.8.8.0\t8.8.8.255\t15169\tUS\tGOOGLE\n"
+    # ASN 0 sentinel — must be skipped.
+    "100.64.0.0\t100.127.255.255\t0\tNone\tNot routed\n"
+    # Malformed addresses — skipped.
+    "garbage\tnonsense\t12345\tXX\twhatever\n"
+    # Reversed range (end < start) — skipped.
+    "10.0.0.10\t10.0.0.5\t99999\tXX\tBackwards\n"
+    # Valid row with empty description.
+    "46.101.0.0\t46.101.255.255\t14061\tDE\t\n"
+)
+
+
+def test_parse_plain_tsv(tmp_path: Path) -> None:
+    fixture = tmp_path / "ip2asn-v4.tsv"
+    fixture.write_text(_FIXTURE_TSV)
+    ranges = list(parse_file(fixture))
+    asns = {r[2].asn for r in ranges}
+    assert asns == {13335, 15169, 14061}
+
+
+def test_parse_gzipped(tmp_path: Path) -> None:
+    fixture = tmp_path / "ip2asn-v4.tsv.gz"
+    with gzip.open(fixture, "wt", encoding="utf-8") as fh:
+        fh.write(_FIXTURE_TSV)
+    ranges = list(parse_file(fixture))
+    asns = {r[2].asn for r in ranges}
+    assert 13335 in asns and 15169 in asns
+
+
+def test_parse_range_boundaries(tmp_path: Path) -> None:
+    fixture = tmp_path / "ip2asn-v4.tsv"
+    fixture.write_text(_FIXTURE_TSV)
+    ranges = [r for r in parse_file(fixture) if r[2].asn == 15169]
+    assert len(ranges) == 1
+    start, end, info = ranges[0]
+    assert start == int(ipaddress.IPv4Address("8.8.8.0"))
+    assert end == int(ipaddress.IPv4Address("8.8.8.255"))
+    assert info.name == "GOOGLE"
+
+
+def test_parse_empty_description_kept(tmp_path: Path) -> None:
+    fixture = tmp_path / "ip2asn-v4.tsv"
+    fixture.write_text(_FIXTURE_TSV)
+    ranges = [r for r in parse_file(fixture) if r[2].asn == 14061]
+    assert ranges[0][2].name == ""