Account progression staging

Assigns any account a single current progression stage from its full signal stack, records the stage entry with the evidence that triggered it, and routes an alert when an account crosses into Sales Ready. Progression is account-level and buying-group-gated: the question is never "did one person do something" but "is this buying group moving forward." Every run updates the running stack in account memory and writes exactly one stage tag, so the stage is always defensible from the memory snapshot alone.

The point of staging this way is measurement. Once accounts carry a stage and a stage-entry history, "attribution" stops being touchpoint credit and becomes delta versus baseline — did exposure to a campaign or channel raise the conversion rate or shorten the time for a stage transition, versus a comparable unexposed cohort. This skill produces the per-account substrate that makes that measurement possible; the measurement layer at the end covers the cohort math.

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