steps callers: the runner, the indexer, the operator and sessions (D-18); BLD12 builds in the fetch;
BLD13 resolved and the toolchain digest; BLD14 the step key <asker>.<node>.<id> beside main's
<node>.<id>; the events toolchain-reported and step-progress on the three build seats, and
step-progress on mesh-wants.
Builds: design 47 G §463, §480, §465, §466, §467, §481, §483, §492, §496
Claude-Session: https://claude.ai/code/session_01JG73KEq4zoDfM9WtYpcQgZ