docs(plan): per-year population denominator implementation plan
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.
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