Repository-owned Bridgeland foundations

Stable owner-authored root data for filtrations and stability conditions.

Import policy

This file is the complete downstream umbrella and is not an internal prelude. Implementation modules import the narrow owner of the declarations they use:

• Foundation.Slicing for the root slicing and HN-filtration structures, with individual Foundation.Slicing.* leaves for later consequences;
• Foundation.PreStabilityCondition, Foundation.IntervalCategory, and Foundation.StabilityCondition for their respective stable layers;
• individual Foundation.StabilityFunction.* leaves for the abelian HN API;
• individual Foundation.Deformation.* leaves for deformation-theoretic results.

The foundation-import-boundary gate prevents implementation modules from importing this umbrella and silently acquiring all of those layers.