v0.18.4

Fixed

• --allow-destructive no longer drops a declared unique constraint as
  collateral. A single-column unique constraint declared in a table's
  indexes: as unique: true + as_constraint: true was correctly treated as
  unchanged by a plain plan/run, but a run --allow-destructive (needed for
  any legitimate DROP TABLE/DROP COLUMN) silently scheduled that same
  constraint for removal — allowing duplicate rows afterward on a populated
  database. The additive and destructive planners now agree: a constraint
  declared in YAML is never classified as extraneous, regardless of run mode.