A startup race between `decnet bus` being ready and the API's lifespan
hitting `get_app_bus()` at api.py:135 would set `_tried = True`
permanently, poisoning the singleton for the rest of the process: the
dashboard shows BUS OFFLINE, topology SSE falls into the bus-is-None
snapshot-only branch, mutator publish calls no-op. Only an API
restart recovered.
Replaces the one-shot veto with a time-gated retry keyed on a
`_last_failure_ts` monotonic timestamp plus a 2 s backoff. Publishers
on the hot path still pay at most one connect attempt every 2 s when
the bus is down, but the singleton auto-recovers within 5 s (one
dashboard poll) once the bus comes up.
The asyncio lock still serialises concurrent callers so the bus server
doesn't get stampeded with parallel connect attempts on startup.
