Protocol 6 amended in place, so PROTOCOL_VERSION is unchanged. Six routes and a
fourth responder; no store migration and no new machinery.
`event_call` is `admin_call` without the required `actor`: an event verb's author
is a RUN, which the body carries as `runId`, and demanding a human name for
something no human is doing would have the runner inventing one.
`respond_event` exists for two mappings the generic responder gets wrong. A
drifted lease is a 200 -- the shard was asked to compare and set, it compared,
and it declined to overwrite somebody's deliberate change, which is the mechanism
working -- and deliberately not the 409 the version gate owns, for the same
reason 425 is not. And the event plane being switched off is a 403 rather than a
reason-sniffed 400: it is an operator's deliberate refusal, and a 400 would send
an administrator hunting a bug in a step that is written correctly.
`participation.snapshot` is a POST for a read, because it carries the caller's
idempotency key and the shard may refuse it as a repeat in flight.
Co-Authored-By: Claude <noreply@anthropic.com>
