maxPayloadBytes

The base's budget less the relay header this overlay adds (#2058).

Held back unconditionally, even though only one of the two send paths pays it: broadcast wraps the payload in a GossipFrame so receivers can dedup and re-flood it, while sendTo passes through to base unwrapped. Seam.maxPayloadBytes settles which of the two the published number follows — "subtract unconditionally, even when the overhead is only paid on some routes", because a budget that moved with the route is a TOCTOU trap for a caller that reads it and then sends.

(#2058's own table filed this decorator under "adds no bytes". It does; the wrap above is it.)

null stays null — a base that names no ceiling has told this overlay nothing — and the result is floored at zero, so a base tighter than the relay header publishes 0 rather than a negative budget. Read through per call: the base's number moves.