Composition leaves were rendered flat — the cascaded inner derived
templates existed in the DB but the tree only showed the outer slot
name (e.g. "Tank Monitor > DrivePump") with no way to see DrivePump's
own TempSensor + AlarmSensor slots.
BuildCompositionLeaves now recurses: for each composition under a
template, look up the composed template (which after derive-on-compose
is a derived row carrying its own Compositions) and build its slot
leaves as children. HasChildrenSelector loses the
"not a composition" guard so nested leaves render with the expand
chevron.
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