Should *_would_be_qualified columns be derivations rather than QRF targets?

The Build M base jetsam (fixed mechanically in #415) exposed a modeling question worth an architecture-week look.

The QBI family (estate_income_would_be_qualified, farm_operations_income_would_be_qualified, farm_rent_income_would_be_qualified, partnership_s_corp_income_would_be_qualified, rental_income_would_be_qualified, self_employment_income_would_be_qualified, sstb_self_employment_income_would_be_qualified, plus business_is_sstb) are 3–6-atom near-categorical columns imputed through the primary PUF QRF chain. A quantile forest is a strange model for a six-level ratio: it can only reproduce the atom set via leaf quantiles, the atoms look like they encode qualification-rule levels applied to the base income column, and the chain pays a full forest per target for what may be a deterministic (or simple conditional-categorical) relationship to columns drawn earlier in the chain.

Questions for the review:

1. What defined these columns in the retired us-data pipeline — a rule over the base column, or independent survey/PUF signal? (Chesterton fence: find the origin PR before changing anything.)
2. If rule-derived: they belong in the deterministic-derivations stage (populace#395 canonical ordering), run once on the assembled multispine, not per-source QRF.
3. If genuinely stochastic: a categorical draw (per-level gate) is cheaper and better-specified than a QRF over atoms.

Not launch-blocking — #415's leaf bound makes the current model tractable and Build M proceeds under it. Filing so the modeling question doesn't vanish behind the memory fix.

🤖 Generated with Claude Code

Start by locating the origin PR and the retired us-data pipeline definitions for the listed *_would_be_qualified columns. Then review the deterministic-derivations stage described in populace#395 and the primary PUF QRF chain; done means documenting whether these columns are rule-derived or stochastic and selecting the appropriate modeling path.