test: the dispatch generator has no family for LinuxSLL's ARPHRD registry #2592

LinuxSLL.__proto__, the ARPHRD-keyed registry from #2510/#2527, has entries for FRAD, IPGRE, IP6GRE, IEEE80211_RADIOTAP and NETLINK, but no Family in examples/generators/dispatch.py. The FRAD probe that #2560 adds reaches LinuxSLL's FRAD entry only indirectly, and nothing generates the others. test-coverage.rst's protocol row asks for a PINNED_TARGETS entry per __proto__ registration, but it does not say that a protocol with a registry of its own (FRAD, Loopback, LinuxSLL) also needs a Family. That gap is how #2560 happened.

Fix: add a linux_sll family covering every LinuxSLL.__proto__ entry, with PINNED_TARGETS, and bump the count guard in test_cases_cover_every_table_named_in_the_issue. Add the missing clause to test-coverage.rst's protocol row.

Blocked: #2560's branch, #2575 (radiotap) and #2489 (DCCP) all append to dispatch.py's FAMILIES/PINNED_TARGETS and to that count guard. This issue unblocks when their PRs merge (gh pr view <n> --json state -q .state prints MERGED).