Unique-index coverage as an alternative to PK for mutation cardinality safety

R144’s PK-coverage check on DELETE / UPDATE filter columns is the conservative cut: PK is the only unique key the validator considers when deciding whether to require @multiRow. Real schemas often have alternate unique constraints (e.g. a natural-key column with a unique index alongside a surrogate PK), and forcing those mutations under @multiRow discards the per-row cardinality guarantee the unique index would otherwise provide. This item generalises the check from "covers PK" to "covers some unique key" by exposing unique-index metadata on JooqCatalog and threading the matched-key identity through to the lookup-WHERE emitter (so RETURNING / follow-up SELECT know which columns to round-trip). Independent from R145’s UPSERT story; either can ship first.