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

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 #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.

🤖 Generated with Claude Code

Start with the dynamic-layer reform-scoring work and the current-law certification chain referenced by #113; this issue depends on those scored person-level paths. Compare the existing year-by-year and income-band views, then define the birth-cohort aggregation for survey-frame data, including aggregate and per-capita variants. Done means the cohort view appears in the public dashboard alongside the benchmark wall after the underlying quantities are gate-certified.