The chain's POOLED credit headroom — one grouped query over the TOPMOST capped node's subtree (joining the credit
ledger via the credit_key sidecar), then pooledHeadroom. This is where the abuse-proof cap becomes real: a
parent's cap bounds its whole subtree's spend. Null when no node in the chain declares a cap (uncapped).
The chain's POOLED credit headroom — one grouped query over the TOPMOST capped node's subtree (joining the credit ledger via the
credit_keysidecar), then pooledHeadroom. This is where the abuse-proof cap becomes real: a parent's cap bounds its whole subtree's spend. Null when no node in the chain declares a cap (uncapped).