feat(internet): implement IGMP v1/v2 (v3's group-record list as a follow-up) #2439

Is your feature request related to a problem? Please describe.
Wave 2 of the roadmap. IGMP has no implementation -- grep across pcapkit/ finds only protocol-number constants (transtype.py's IGMP = 2, apptype/udp.py's igmpv3lite) and the Mobility Header module carrying IGMP group records as opaque bytes, with its own docstring saying explicitly they belong to IGMP and are "carried opaquely here" for lack of a real IGMP dissector to hand them to. pcapkit/protocols/internet/__init__.py:12 lists IGMP in the maintainer's own standing TODO.

Describe the solution you'd like
IGMP v2 (RFC 2236) first, as an Internet subclass (schema/data/protocol split, per discussion #251). v1 (RFC 1112) is a strict subset of v2's 8-byte header -- same shape, fewer message types, no max-response-time field -- and is largely obsolete in the wild, so it's a near-free addition once v2's header is in; v3 (RFC 3376) adds a materially different group-record-list body for source-specific multicast substantial enough to warrant its own follow-up rather than blocking this PR.

Describe alternatives you've considered
v3 first since it's a superset in spirit -- rejected: the group-record list body is a different enough shape that building v2's simpler header first, then extending, is less churn than building v3 and retrofitting v1/v2 compatibility onto it.

Additional context
IGMP's message-type values (0x12/0x16/0x22) are already referenced opaquely in mh.py's mobility-header carrying logic -- landing v1/v2 here gives that registry something real to dispatch records to, ahead of the Mobility Header sub-layer registry work pep.rst sequences behind wave 2.