- CAMPAIGN_COUPON_AB_PCTS="20,30,40" mints one daily code per arm; each
carrier is bucketed by a stable sha256(email) hash so the split is even
(~33/33/33 verified over 30k) and stable across re-sends (no arm-hopping).
- Each arm's code stores its own percent in discount_codes, so the advertised
discount always matches what checkout applies; redemptions are countable per
code (marker campaign-daily:<date>:<pct>).
- Empty/unset keeps legacy single-arm behavior (COUPON_PCT, legacy marker).
- coupon_attribs() now takes per-recipient pct.
- Tests: scripts/tests/test_coupon_ab.py (5 pass). SpamAssassin: both main
campaigns (186/188) score 0.0 HAM across all 3 arms, coupon block renders
clean; harness saved for re-runs.
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