AddCompositionAsync creates a derived Template ("<parent>.<slot>") that
inherits from the base via ParentTemplateId. Base attributes and scripts
are copied with IsInherited=true so the derived template carries its own
override-able rows. The composition row points at the derived template,
and the derived's OwnerCompositionId back-refs the composition for cascade
delete.
DeleteCompositionAsync cascade-deletes the owned derived template.
DeleteTemplateAsync blocks direct deletion of derived templates and
distinguishes derivatives from regular children, listing slot owners
("'Pump' (as 'TempSensor')") in the error.
Composing a derived template is rejected — only bases can be composed.
Existing compositions still resolve until phase 3 migrates them.
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