Add a third reason to the unresolved-faminst placeholder explanation

Add a third reason to the unresolved-faminst placeholder explanation

The user observed cases where a more general fam-instance LHS
(e.g. `data instance LedgerState (ShelleyBlock proto era) mk`)
should cover a more concrete use site (`… EmptyMK`) but the chain
doesn't draw — biUnify's wildcard rule on TyVarRef should handle
this in principle, but kind-arg interactions in the rep TyCon
can leave the unifier looking at structure that doesn't quite
line up. List this as a third reason so the side panel stops
implying the only failures are external-family / nested-args.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>