`get_health_ready` computed "degraded iff any warning is crit" inline and
wrapped it in a private `ReadyBody`. The swarm status publisher needs the
same verdict, and the warnings module's own doc already states why it must
not compute its own: two systems independently deciding what counts as
unhealthy is how they end up disagreeing.
The disagreement would also be silent. Each side would look internally
consistent, and the day a second degraded condition is added to one of
them, the dashboard and the swarm view would report different things about
the same host with nothing to flag it.
`warnings::readiness()` is now the single producer and `Readiness` the
single type. `ReadyBody` is deleted rather than made public: the endpoint
keeps the part that genuinely is its own, the mapping onto an HTTP status
code, and serves the shared document as its body.
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