Show whether hotspots and module cohesion improve over time #19

Context

A single analysis window shows the current state, not the direction. After a refactoring, the useful question is whether a module became more cohesive and whether hotspots cooled down.

Goal

codeheat compares two adjacent windows (for example the last 3 months against the 3 before) and reports how hotspot scores and module cohesion changed.

Acceptance criteria

• A flag such as --compare <duration> produces per-file and per-module deltas in the JSON (additive fields) and in the terminal view.
• The treemap can color by change instead of by current heat.
• Engine tests with a history that improves in the second window prove the sign and size of the deltas.

References

• Part of Roadmap: judge modularization and module depth #20.