Builds the frozen validation corpus: samples postcodes from the register, then
caches each postcode's full cohort of raw cert payloads (the shape
from_api_response consumes), grouped by postcode, resumably. Reads the token
from backend/.env; cache dir /tmp/epc_prediction_corpus (EPC_PREDICTION_CORPUS
override). IO plumbing, not test-driven. Pairs with the leave-one-out harness.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>