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.

.Rbuildignore

@@ -11,3 +11,4 @@
 \.gitkeep$
 ^vignettes$
 ^specs$
+^plans$