Reduce codebase by implementing top five refactoring

Identify top 5 opportunities to massively reduce the amount of code (lines of code) in any part of the repo: - Similar functionality that can be unified and reused - Code that has been valuable but no more - A front end component or a group of them duplicate another component/page functionality. Use the most modern and nice looking - etc. Now implement them :)

Shipped PR #6895 (queued to the merge lane): five behaviour-preserving consolidations, 48 files, +230/−617 (api twins, shared count_by, dead code ×3, jaahd marker/uid twins, shared test loaders). Quick gate and all touched suites green. Repo measured at its mechanical floor; migration re-baseline (~8K lines) named as the one large lever, operator-cut. Memory: used mem-25288/14612/18424; proposed 1 gotcha.