When the user composes a template that already has compositions of its
own (e.g. \$Sensor → Probe1 slot), only the outer derived was created
— the source's children weren't replicated. AddCompositionAsync now
walks the source's composition graph and creates a parallel derived for
every slot it encounters, each linked back through ParentTemplateId so
override chains stay intact (\$Probe → \$Sensor.Probe1 → \$Pump.TempSensor.Probe1).
The cascade pre-flights every name it would create — a deep collision
aborts before any rows mutate. Internal helper
CreateCascadedCompositionAsync skips the "base templates only" check
since it operates on the source side which may legitimately reference
derived rows.
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