0624 — the first RegisterTopic stands up the control-plane schema

Context. Every program had to call RegisterSystem before its first RegisterTopic, yet for most of them the call carried nothing: SystemConfig has no fields, so the only content in RegisterSystemConfig is the two built-in alert schedules, which almost nobody sets. The quickstart and every playground carried a line whose only job was avoiding VK0017.

Decision. Public RegisterTopic checks for the system row and, when none exists, runs RegisterSystem(ctx, nil) before registering the topic. The check is register-if-absent, never a re-declare — a system registered with custom alert schedules keeps its declaration, because nothing runs when the row exists. RegisterSystem stays public as the path for setting cfg (and as MigrateSystem/DestroySystem’s sibling); every other verb keeps its VK0017 gate unchanged, including the internal registerTopic gate as a backstop. Topic catalog reads map 42P01 to absence the way scanSystemData already does — GetTopic returns (nil, nil) and ListTopics returns empty against an unregistered database, so the first error a misordered program sees is VK0005 (“register it with MessageAdmin.RegisterTopic first”), not a raw undefined-table error.

Consequences. The quickstart’s samples and six playgrounds lose the RegisterSystem line; scenario 03 loses the whole admin detour. First boot is concurrency-safe without new locking: system.register already serializes table creation and the singleton seed under pg_advisory_xact_lock, and a probe racing six pools calling bare RegisterTopic on an empty database converged every round. The implicit register costs one system_config SELECT per RegisterTopic call.