Cohort migration tracker: which countries are shifting DTW censorship cohorts over time

Voidly Atlas already clusters the 50 highest-signal countries into DTW cohorts — C1 stable democracies, C2 bursty, C3 persistent authoritarian — by daily-signal SHAPE similarity under Dynamic Time Warping. But that clustering was a one-shot snapshot: it told you where a country sat, never whether it had moved. The cohort migration tracker closes that gap. It recomputes the cohorts on a rolling 90-day window every month, compares the new assignment to last month's, and emits the list of countries that crossed cohorts — with a direction-of-travel label (deteriorating / improving / lateral) and a per-country fit confidence. A country sliding C1→C3 is one of the cleanest leading indicators of a censorship regime change we can compute. The hard part is stable cohort identity: re-running clustering reassigns arbitrary cluster IDs, so the tracker gives each cluster a stable semantic label from anchor-set overlap (CN/RU/IR/MM pull toward C3; US/GB/DE/FR/CA pull toward C1) plus centroid burstiness. First snapshot pair (90d-now vs 90d-30d-ago, both from real evidence): 6 countries changed cohort. Four deteriorated C1→C3 — Nigeria (conf 0.53), Zimbabwe (0.51), Mexico (0.50), Slovenia (0.46) — and two improved C3→C1: Iraq (0.68) and Venezuela (0.66). Honest caveats baked into every response: DTW silhouette is only ~0.38 (modest separation, cohorts overlap), a cohort shift can be data-driven rather than regime-driven (treat as a signal to investigate not a verdict), and the monthly cadence will miss faster transitions. Live at GET /v1/atlas/cohort-migration; per-country history at /v1/atlas/cohort-migration/{cc}; rebuilds monthly on the 1st at 06:00 UTC.