Readouts no longer take a lease's connection slot: the accept loop admits
max_connections + READOUT_CONNECTIONS, a lease holds one of max_connections
after it is read, a readout one of READOUT_CONNECTIONS, and a readout over its
limit is refused Busy rather than dropped. Socket tests hold the service lock
and a metered cell across readouts (no asking, no week wait, no lock), flood
the readout pool beside leases, and pin the refusal. The new public request
type is LeaseUsageReadoutRequest so it no longer shares a name with the kit's
EnvelopeKind::UsageReadoutRequest; wire bytes and fixtures are unchanged.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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