Aveva-style composition: composing $Sensor into $Pump creates a
derived template Pump.TempSensor that inherits from $Sensor and can
override values, override script bodies, add new fields, with
LockedInDerived on the base preventing specific overrides.
Schema sketch: Template gains IsDerived + OwnerCompositionId;
TemplateAttribute/Script gain IsInherited + LockedInDerived.
TemplateComposition.ComposedTemplateId pivots to point at the
derived template (the base is reachable via derived.ParentTemplateId).
Phased rollout (9 phases), starting from additive schema, then
flow change for new compositions, then EF Core migration of
existing data, then resolution, lock semantics, tree UI, derived
template edit UI, base template lock-toggle UI, editor metadata
simplification (multi-parent picker becomes mostly obsolete —
derived templates always have a single owner).
Open questions captured at the end for review before phase 1.
