0363 — Register validates the topic handle fail-fast: by-name fetch plus whole-struct compare

Context. A producer holds a *topic.Topic handle that can go stale: the topic row can be deleted, destroyed-and-recreated (the handle then addresses dropped tables), or altered. Failing at Register beats failing at first Produce.

Decision. Register does a name-keyed GetTopic and compares the whole struct against the held handle. Three distinct outcomes: row gone returns topic.ErrTopicNotFound; id changed returns topic.ErrTopicStale (destroyed and recreated — the id rides in the struct, so the by-name fetch catches it); any other field drifted also returns ErrTopicStale. Whole-struct compare settled the “which fields matter” question: a stale handle is a stale handle.

Consequences. No partition cold-start work at Register: the producer’s 23514 self-heal is synchronous with the insert that needs it, so a Register-time ensure would only shave one round-trip once per process. Revisit with evidence — a producer-only topic pays the heal at every partition boundary.