Studio Stability— cross-run rule classification

Compares the last N studio run artifacts and labels each (path, scope) tuple in the combined patch as STABLE (same value across every run that included it), DRIFTING (monotonic up or down — possible genuine signal), VOLATILE (oscillating — likely overfit to recent data), NEW (only the latest run), or DROPPED (was recommended, isn't anymore). Studio recommendations are advisory — review the STABLE set first; skip VOLATILE; treat NEW with no track record.