E2 era-classification probe

Year-from-text (FineWeb sample-10BT): acc 0.2283 vs majority baseline 0.125 over 8 years
within 1 yr: 0.4135, within 2 yr: 0.618, mean |err|: 2.267
selfanchor density by year: {"2013": 0.5877, "2015": 0.5825, "2016": 0.5725, "2017": 0.4523, "2018": 0.4329, "2019": 0.4172, "2020": 0.4279, "2022": 0.4154}
C4 URL-year heuristic (E1b): 19.37% of docs carry a path year