PROTOCOL_VERSION 5 -> 6, and one behaviour: `bridge.busy` maps to 425 Too Early in
all three responders.
Everything else is free, and that is the point. The key rides in the command body,
which every write endpoint already passes through verbatim; `champ.boss.killed`
lands in `events` and on the feed through the generic forward path with no arm of
its own. No store migration — nothing gains a column.
Worth naming what the dumb-forwarder property means here specifically: the sidecar
makes no idempotency promise of its own. It does not dedupe, does not cache, and
does not know what a key means. The guarantee is the shard's, end to end, which is
the only place it can be.
425 rather than 409 because 409 is already the protocol-version gate's answer, and
the two want opposite dispositions from a client: a version mismatch is a
deployment fault nobody should retry, a busy shard is a retry that will succeed.
Sharing a status would make the difference readable only by inspecting the body,
which is how a retry loop ends up hiding a mismatched deployment. A replayed reply
is an ordinary 200 — `replayed: true` is for the log.
Co-Authored-By: Claude <noreply@anthropic.com>
