Deepen a lived-in codebase's confirmed hot modules without changing behaviour: a churn-weighted shallowness inventory finds the modules whose interfaces are too wide for what they hide, a human confirms the target surface, a mutation-audited characterization net (prove-it's, consumed here) is pinned BEFORE any restructure, and each module is deepened one seam at a time with build-blind review. The unit is one module-deepening (one interface shrink at one seam). Use when "this module is a god file", "the interface is wider than the implementation", "too many imports to change anything", "architecture erosion", "refactor the hot path safely", "shallow modules", "deep module…
