Hosts first, then the controller — a build and a push each, now that the
mesh delivers the host. The host refuses a lower sequence than it kept
and drains a batch by sequence rather than arrival; the controller
numbers each send under the node's hold, inside the signed bytes.
Measured: two pushes, sequence 2 in the kept declaration, counters in
the store agree, no machine reads as behind. That last one is the
subtlety: the mesh compares the digest of what it would send against
what it did, and a number changes the bytes, so the read-only comparison
composes with the last number sent rather than a fresh one.
