docs: implementation plan for cog_balances() (#25)
Six TDD tasks: the two views + registration gate, the core verb, per_capita and adjust_to_year, recipe=, balance_caveats provenance, docs. Every internal the plan calls was verified to exist with the signature used (.build_verb_sql, .shape_recipe_result, .attach_per_capita, .run_recipe, .require_schema_v5, ...), so the tasks reuse the shared machinery rather than reimplementing it. The verb deliberately does not route through .verb_spendrev(), whose concept scoping, IG leg and complete= grid are all flow-specific. Test government is ALABAMA STATE GOVT (010000226085), which covers every case in the bundled fixture: W01/W31/W61 in 2012/2019/2020, X21+Z77 in 2012, Y07/Y08 throughout, and X40 in 2011 -- so the wide-era recipe bridge is testable offline.
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