Get Company Wall Summary

Cross-org Effort band: populated entries summed field-wise.

Each stat's delta is recomposed from the summed current + prior windows (prior = value / (1 + delta) per org, exact because ImpactStatInfo.from_window nulls the delta only when prior <= 0) — summing the per-org fractions directly would double-count. An org with delta == -1 (current 0, prior magnitude lost) makes the pooled delta unrecoverable, so it reports None instead.