DeleteCompositionAsync only dropped the top-level derived template — the
cascaded inner derived rows (created when composing a composite source)
were left orphaned with dangling OwnerCompositionId references. Any
subsequent attempt to recompose the same source hit the name-collision
guard ('Motor Controller.Pump.TempSensor' already exists).
New CascadeDeleteDerivedAsync walks each composition on the derived
template, recursively removes the slot-owned child derived first, then
the composition row, then the derived itself. Mirrors the recursive
shape of CreateCascadedCompositionAsync.
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