Review round 1: the decoder type ships before any composer can emit it, so a
routine SDK bump never sends yields to an engine that refuses them; the
composer half moves to a later change. Every yielding process of a node goes
into one top-level slice, meshyield.slice, so the three build modules share one
quota instead of taking one each; the share is one node value, node-tools'
build-cpu-share, and each step's threads divide it by every build slot of the
node (words from the composer, a conservative fallback without them).
jschoubben
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