0542 — Config & options refinement: the three shape decisions

Context

The 14b config pass opened with a survey: across all 48 Config structs the required/optional split already held — every field either WithDefaults- filled or zero-meaningful — except PostgresConnectionConfig, whose User/Host/Database were required fields hiding in a config. The open decisions were the OptionalConfig rename, the ProduceOptions compaction shape, and Consumer.Register’s five params.

Decision

• Config keeps its name; the rule is codified in CONVENTIONS.md instead: a Config struct holds ONLY optional fields, and a Validate error on a field WithDefaults never fills marks a required value to move into the constructor’s params. PostgresConnectionConfig fixed accordingly: NewPostgresDatastore(ctx, user, host, database, cfg); the config keeps Pass/Port/MaxConns/ConnectTimeout/TLSConfig.
• Compaction nests (user-picked over flat + Validate): ProduceOptions drops CompactionKey/CompactionRank for one Compaction *CompactionOptions {Key, Rank}, built with NewCompactionOptions(key, rank) — rank 0 means arrival order. Rank-without-key is now unrepresentable; nil means not compacted (the MessageOptions nilable precedent, [0536]). The defer-requires-key rule survives as defer-requires-Compaction. Table- exact row structs and AppendData keep their flat compaction_key/ compaction_rank columns — the nesting is produce-surface only.
• Consumer.Register keeps its five params (the four required mirror Producer.Register plus group; bindings stays the one nil-able optional); NewConsumerInstance unexported instead — only Register called it, so the 7-param public signature just disappears.

Consequences

• A maybe-keyed produce path now branches to build its options (labs that published unkeyed baselines through a keyed helper grew that branch) — the branch is the design making “not compacted” explicit.
• CLI conn.go owns a connection parse-result struct; missing user/host/ database in a URL fail as usage errors at parse, and the constructor backstops embedders at dial.
• Verified: all six module trees build + vet, root tests pass, and compactionlab, compactionranklab, cronlab, deferlab pass live.