0245 — Topics are registered explicitly and never auto-created on use

Context. A topic is a durable resource commitment: registering one constructs a physical message_log_<topic_id> table and locks in configuration (sequence, partition set, retention settings), some of it immutable.

Decision. Topics exist only via explicit topic.Register. Producing to or claiming from an unregistered topic id fails with a clean Postgres 42P01 undefined_table error — it never silently creates the topic. This is proven end to end in topiclab.

Consequences. Creating a topic is a deliberate moment, not an incidental side effect of a produce or claim call — a typo cannot fork a whole new physical log into existence. This is the same weight argument that kept routing_key (free text, zero ceremony) as a separate concept above topics (0242). Partitions, by contrast, keep self-creating via EnsureNextPartition: their names are computed from id / partitionSize, never caller-supplied, so no typo can fork one.