refactor(protocols): every versioned family base dispatches on read and make (IGMP, IP, ICMP, L2TP, LinuxSLL) #2574

Standing convention from the owner on #2544: on a versioned protocol family, the base class is a dispatcher on both the read and the make path. Parsing bytes through the base, or building through it (constructor or make), yields the matching member, never a base-class layer. #2544 applies it to HTTP (HTTPv1/HTTPv2) and writes it onto protocol-family-grouping.rst.

Families to bring into line, those protocol-family-grouping.rst lists with versioned members:

• IGMP (IGMPv1–3). Its base is abstract today and dispatches only from Internet._import_next_layer.
• IP (IPv4/IPv6).
• ICMP (ICMPv4/ICMPv6).
• L2TP (L2TPv2).
• LinuxSLL (LinuxSLL1/2).

For each, check what the base does now on direct construction from bytes and from fields. Make both paths yield the member, mirroring #2544's mechanism, and keep byte-exact round trips. Then test each family, and show the tests fail before the change.

Blocked: this mirrors #2544's mechanism and edits files #2542 (IGMP), #2527 (LinuxSLL) and #2544 change. It unblocks when #2544 merges; IGMP and LinuxSLL also need #2542 and #2527 merged first.