Model-level isFkHop/pairsOf helpers for JoinStep narrowings

R438’s self-review cleanup (1), spun out of R431 per its spec’s explicit option ("splitting it to a follow-on item is an acceptable outcome if this item runs long"; R431 slice 4 landed the coupled cleanup (2), the bridging-join consolidation into JoinPathEmitter). The FK-hop narrowing idiom, instanceof JoinStep.Hop h && h.on() instanceof On.ColumnPairs, plus blind (On.ColumnPairs) hop.on() casts, appears inline roughly forty times across ~16 main-source files (re-count at pickup: grep -rn "instanceof JoinStep.Hop" graphitron/src/main and "instanceof On.ColumnPairs"), each a hand-rolled repetition of one model question ("is this hop FK-derived, and what are its column pairs"). A model-level isFkHop(JoinStep) / pairsOf(JoinStep) helper pair (natural home: JoinStep or a static on On) replaces the inline narrowings; the exhaustive sealed-switch sites are proper dispatch and stay. TestFixtures.isFkHop already exists as a test-side copy of the predicate and should fold onto the model helper. Acceptance mirrors R431’s: generated output byte-identical, the diff audited in isolation; full reactor green.

Scope note (2026-08-19, revised 2026-08-21): R705 changes this item’s census in one way, not the two an earlier version of this note claimed. It retires FkHop outright, whose narrow was one of the census’s members, and introduces one new hop-typed carrier of its own (ReachPath, whose compact constructor is the single narrowing site for the condition filter rail).

It does not lift the declared type of the path-carrying components. An earlier R705 draft lifted four of them (ParsedPath.elements, BodyParam.RemoteColumnPredicate.joinPath, and the joinPath components of ArgumentRef.ScalarArg.ColumnBackedArg and InputField.ColumnBackedReferenceField) from List<JoinStep> to List<JoinStep.Hop>; that was dropped at R705’s third Spec review, because Java generics are invariant and ParsedPath.elements alone feeds 47 read sites, so the lift is a compile error at every consumer that has not lifted with it. The reasoning is in R705’s Review resolutions section.

So the instanceof JoinStep.Hop narrowing sites on those paths survive R705 and remain this item’s census. Whether the right consolidation is the helper pair or the type lift is still this item’s call, and the lift’s real cost is now measured: see R705 for the site counts. Re-count at pickup as already instructed above, after R705 lands if it is in flight.