15-task TDD plan covering: gov_population_yearly view, .attach_per_capita
per-year join, pop_source column + multi-note concatenation, cog_find_peers
year arg, cog_peer_compare cohort_year, rollup unavailable-pop exclusion,
provenance updates, cog_explain rendering, vignette, cog_pipeline data
dictionary, and NEWS entry.
Also corrects spec to match existing rollup semantics (side-by-side, not
summed) and adds /plans to .Rbuildignore.
jared