EF migration MigrateCompositionsToDerived. Aborts with a clear error if
any '<parent>.<slot>' derived name would collide with an existing
template. Otherwise it cursor-walks every TemplateComposition that still
points at a non-derived template:
1. Insert a derived Template (name "<parent>.<slot>",
ParentTemplateId=base, IsDerived=1, OwnerCompositionId=composition).
2. Copy base attributes / scripts into the derived row with
IsInherited=1, LockedInDerived=0.
3. Repoint TemplateComposition.ComposedTemplateId at the new derived.
Idempotent: only touches compositions whose target is IsDerived=0, so
re-runs and freshly-created Phase 2 compositions are skipped.
Down() reverses by repointing compositions back to derived.ParentTemplateId
and dropping all derived templates (with cascade copy rows).
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