refactor(link): dispatch LinuxSLL's non-EtherType ARPHRD values through its own registry map #2510

Owner ruling (this session, 2026-11-01), paraphrased: the link dissectors LinuxSLL dispatches by arphrd_type should index by their ARPHRD value, and LinuxSLL should hold those values in a registry map of its own instead of a hard-coded branch chain.

Scope

• FRAD, GRE and NETLINK: __index__ returns the matching pcapkit.const.linux.arphrd.ARPHRD member (each currently raises).
• LinuxSLL overrides __proto__ with its own map keyed by the non-EtherType ARPHRD values, as FRAD already does for NLPID. _import_next_layer dispatches through that map first, then hands every other value to super()._import_next_layer for the EtherType path. The _ARPHRD_NOT_ETHERTYPE branch chain goes.
• LinuxSLL, FRAD and NETLINK each define their own register classmethod. Today FRAD and NETLINK inherit Link.register, which is EtherType-typed.
• The new map gets its register classmethod and a register_* wrapper in pcapkit.foundation.registry.protocols, shaped like register_ethertype. The same applies to the FRAD.__proto__ and NETLINK.__proto__ maps (see refactor(registry): every class-level registry map gets its foundation.registry wrapper #2511).
• IEEE80211_RADIOTAP joins the map alongside feat(link): add a radiotap dissector for IEEE80211_RADIOTAP on LinuxSLL #2509; whichever of the two lands second adopts the other.

Open detail: GRE covers two ARPHRD values, but __index__ can return only one. Recommendation: ARPHRD.IPGRE, with both keys registered.