Deformation of stability conditions — umbrella

Re-exports the Foundation.Deformation leaves. At 43 files this is the largest single directory in the library and it had no same-named umbrella, which is why Foundation.lean reached it through 34 flat leaf imports.

This is a re-export for discoverability, not an import target for implementation modules: scripts/check_foundation_import_boundary.py keeps the Foundation umbrella out of implementation modules and directs contributors to a narrow leaf, and the same discipline applies here.