Foundations shared by weak and Bridgeland stability

Slicings and their generic consequences, interval categories, and the heart class datum, all needed before imposing the stronger open-ray and local-finiteness conditions.

The stability functions themselves are not owned here. They are abelian, and MO1.08 (#1319) moved them to CategoryTheory/Abelian/Stability/; this umbrella re-exports that owner so the triangulated weak theory keeps one import surface. HeartDatum.lean is the adapter that instantiates the abelian ClassDatum at the heart of a t-structure, and is the only part of the old Foundation/StabilityFunction/ directory that mentions a triangulated category at all.