Fix equality constraints surfacing every fam-instance

Fix equality constraints surfacing every fam-instance

The instance-view equality edges (added in 9f81486) call
addFamilyLinksFromArgs with predClassQn = null to surface type families
mentioned inside an equality (e.g. `Norm` in `Norm a ~ Int`). But the
relevance filter that keeps only the fam-instances whose LHS matches the
use site was gated behind `if (predClassQn)`, so the null call skipped it
and pulled in *every* instance of the family.

Concretely: drilling into STS instances and filtering down to ShelleyUTXOW
still showed the `State` of every rule, because ShelleyUTXOW's context has
`State (EraRule "UTXO" era) ~ …` and the unfiltered call dumped all ~21
`State` instances.

Apply the relevance filter (replaceFamilyApp unification) regardless of
predClassQn; only the resolution-chain step stays gated on it. Now the
filtered view shows just `State (ShelleyUTXOW era)` plus a placeholder for
ShelleyUTXOW's own unresolved `State (EraRule "UTXO" era)` reference.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>