payloadOfExactlyTheBudgetIsCarried

A payload of exactly Seam.maxPayloadBytes crosses. Sent with broadcast so the obligation does not also depend on SeamCapabilities.supportsSendTo.

This is the edge a fabric gets wrong by publishing a number bigger than its wire really takes — the frame is then refused by the fabric's own machinery, at a limit the caller could not see and did not agree to. null means unknown, so a fabric that names nothing skips: it promised nothing to break.