Filed from the urus v0.3 endpoint work. start_child spawns and moves on,
so a later sibling can whereis an earlier named child before that child's
actor has run. Notes why blocking spawn is not the fix ('has begun
executing' != 'has bound its name', plus a per-accept round-trip tax and
every spawn becoming a context-switch point), that OTP has the same async
spawn and synchronises one level up in gen_server:start_link, the
readiness-ack shape if scheduled, and the structural workaround urus uses
today (registrar spawns its own consumers).
