fix(const): accept RFC 2113's Router Alert 0 and IPX's own socket 0

[JarryShaw]

Fixes #492 — three pcapkit/const/** lookup defects, each fixed in both the generated file and the pcapkit/vendor/** generator that produces it.

1. HIGH — RouterAlert(0), and the root cause is better than the symptom

RFC 2113 §2.1 defines exactly one value, 0 ("Router shall examine packet"), and it is what IGMP, RSVP and MLD put on the wire. RouterAlert(0) raised ValueError.

The obvious fix would have been to add the member. The actual cause is in the crawler: pcapkit/vendor/ipv4/router_alert.py opened with next(reader) # header, but this IANA registry ships with no header row — its first row is the value-0 entry. Verified against the live CSV myself:

$ curl -sL https://www.iana.org/assignments/ip-parameters/ipv4-router-alert-option-values.csv | head -3
0,Router shall examine packet,[RFC2113]
1-32,Aggregated Reservation Nesting Level,[RFC3175]
33-64,QoS NSLP Aggregation Levels 0-31,[RFC5974]

So the generator had been silently discarding RFC 2113's only defined value on every regeneration. Removing that next(reader) and re-running the real entrypoint (python -m pcapkit.vendor.__main__ ipv4.router_alert) against the live registry regenerates the const file with exactly one new member and nothing else changed — which is the proof the vendor half actually propagates, rather than an assertion that it should.

before: RouterAlert(0) -> ValueError: 0 is not a valid RouterAlert
after:  RouterAlert(0) -> <RouterAlert.Router_shall_examine_packet: 0>

A standards-compliant packet now parses: 4600001c...9404000011000000 → options[148].alert == 0.

2. MEDIUM — Socket(0)

0x0000 is an ordinary unspecified socket and IPX's own class default for dst/src, so the class could not construct itself with its documented defaults.

before: bytes(IPX(payload=b'\xde\xad\xbe\xef')) -> ValueError: 0 is not a valid Socket
after:  -> 0000002200000000...  (round-trips)
        Socket(0) -> <Socket.Unspecified: 0>

The vendor side here is a documented workaround, not a root-cause fix, and the PR says so. The Wikipedia table this crawler scrapes has disappeared from the live page independently of this issue, and requests' default User-Agent now gets a 403 from Wikipedia regardless. Both are pre-existing breakages outside this issue's scope. Rather than depend on a scrape that currently returns nothing useful, process() prepends the Unspecified = 0x0000 member unconditionally, with a comment explaining why. Those two scraper breakages are left unfixed and want their own issue.

3. LOW and latent — _missing_ without @classmethod

ResponseKind and GroupingInformation in ftp/return_code.py defined _missing_ as a plain method, so aenum's cls._missing_(value) bound the lone argument to cls:

before: ResponseKind(7) -> TypeError: _missing_() missing 1 required positional argument: 'value'
after:  ResponseKind(7) -> <ResponseKind.Unknown_7: 7>
        GroupingInformation(9) -> <GroupingInformation.Unknown_9: 9>

Every unknown value, not just 0. Fixed in the const file and in the LINE template in vendor/ftp/return_code.py. Rated LOW because these two classes are imported by no runtime code — only the codegen copy — so nothing was broken in practice.

Tests

tests/const/test_const_enum_lookup.py sweeps all 111 IntEnum classes under pcapkit.const, asserting Enum(0) matches what each registry says it should be — 105 accept, 6 correctly reject (ReturnCode and StatusCode are 3-digit codes; ConformanceRequirement, CauseCode, Parameter, MNIDSubtype are registries beginning at 1). That arithmetic reconciles with the pre-fix measurement of 102 accepting and 9 rejecting: the three fixed enums moved across.

It also asserts all 109 _missing_ overrides are classmethods, and parses the RFC 2113 packet end to end.

A second commit, for test isolation

The new sweep passed standalone but failed under full-suite ordering, because tests/cli/test_main.py leaves stubbed pcapkit.utilities.compat/exceptions in sys.modules and this sweep is the first thing to import every pcapkit.const submodule, including ones nothing else touches. Fixed with purge_modules(['pcapkit']) in setUp/setUpClass, the convention every other test file here already uses.

I verified that is a real fix rather than a coincidence — neutering the five purge_modules calls and running pytest tests/cli tests/const/test_const_enum_lookup.py:

3 failed, 6 passed, 4 errors

With the fix, the same command: 13 passed, 111 subtests passed.

Verification

Revert-proof. Restoring next(reader), removing both members and stripping the @classmethods:

9 failed, 2 passed, 107 subtests passed
  SUBFAILED(enum='pcapkit.const.ipv4.router_alert.RouterAlert')
  SUBFAILED(enum='pcapkit.const.ipx.socket.Socket')
  FAILED ...test_every_missing_is_a_classmethod
  FAILED ...test_ftp_return_code_missing_methods_are_classmethods
  FAILED ...test_parses_igmp_over_router_alert_zero

Restored: 7 passed, 111 subtests passed.

Also: test_ipx_unit.py 3 passed; test_ipv4_unit.py 12 passed / 16 subtests; full unit tier 921 passed, 8 skipped, 1859 subtests, 0 failed. mypy on the six touched source files shows only the same 11 pre-existing errors, confirmed identical against the unmodified originals; the new test file is mypy-clean.