Generation-level incidence view: aggregate lifetime net change by birth cohort

[MaxGhenis]

When reform scoring lands on the dynamic layer, add a generation-level incidence view: the aggregate lifetime net change (benefits received minus taxes paid, over each person's full modeled horizon) grouped by birth cohort, alongside the existing year-by-year and income-band cuts.

Motivation: cross-sectional views answer "who bears a change in year Y"; the dynamic frame carries dated earnings and demographic histories per person, so cohort membership is native to the data and "which generation bears the change, in aggregate, across full retirements" is answerable without new modeling machinery — it is an aggregation choice over already-scored person-level paths.

Scope notes:

• Depends on reform scoring on the dynamic layer (post current-law certification chain; see Roadmap: from incidence replication to a DYNASIM-class projection on one unified person-period panel #113).
• Presentation: birth-cohort bins (e.g., 10-year cohorts) with aggregate and per-capita variants; survey-frame first, national weighting when the weighting milestone lands.
• Belongs in the public dashboard alongside the benchmark wall once the underlying scored quantities exist and are gate-certified.

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