Rebuild now runs once per frame via ExecuteAlways Update (OnValidate only
sets a dirty flag + QueuePlayerLoopUpdate), fixing inspector drag stutter
from stacked delayCalls. Adds automatic BoxCollider generation (4 frame
bars, opening slab, optional pedestal box for raised pictures) with an
ownership list so manual colliders stay untouched. Corner radius is
intentionally ignored for colliders.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>