Spec per-lead engagement track

A second sending track alongside the weekly newsletter: engage each lead as it arrives, personalised from its own domain's traffic, search demand, and brand read. Measured the estate first — 1,056,226 leads across 15,397 domains, ~950 arriving daily, and in_campaign=0, so not one has ever been contacted. The existing >=1000-leads-per-domain gate covers 126 domains and is the right trigger for a weekly issue, the wrong one for responding to a signup. Spec is shaped by the constraint that 90% of the list is >2 years cold: fresh-90-days only at launch, subscribed treated as suppression rather than consent, sender split so a bulk send can never take down sign-in codes, and reply handling required before the first send.