Invoked by the Hydrator's Phase-2 instruction dispatch from
a template's adornments field. Instruction applier — clones
each spec's template and attaches it as a fixture via
addFixture; the spec is not retained and there is no paired
getter (the live fixtures are read via getFixtures()). Fixtures
are runtime-only, so this re-runs and rebuilds them on every
hydrate (the BoundaryAnchor reconstruction model).
Public shape added by AdornableMixin.