Metacell size distributions — all 25 combos

Cells-per-metacell histogram for every gene-selection × metacell-generation combination in the rebalanced grid (real production merge/re-split logic, not a fixed target size). Metacell counts range 3,780–6,695 across combos — this shows the actual shape of that variation, not just the totals: most metacells are small (long right tail), and the tail's heaviness differs a lot by method — graph_ward_supercell and agglomerative_ward_full both show a sharp spike of very small (often singleton-ish) metacells that the other three methods don't.