0265 — Optional produce inputs travel in a ProduceOptions struct, not positional strings

Context. Compaction added a second optional per-message input (CompactionKey) alongside the existing routing key, and positional optional strings do not scale or self-document at call sites.

Decision. ProduceOptions{RoutingKey, CompactionKey string} — a sparse struct threaded through Produce → AppendMessage → appendMessage’s INSERT — matching the codebase’s existing *Config convention for optional parameters.

Consequences. Future optional produce inputs extend the struct without breaking call sites; zero-value fields mean “not set” (NULL compaction_key = not compacted). Rejected: positional string parameters — ambiguous at call sites and breaking on every addition.