Show whether the design holds over time: erosion, chronic hotspots, fix density #61

Is the design holding or eroding? Over consecutive windows (quarters across the analyzed period) report change radius, propagation cost and per-module containment, and a trend per module. Classify hotspots as chronic (hot in most windows: design problem) or acute (hot only recently: current work), and report fix density per module (fix commits and reverts, and fixes that span modules).