0612 — the message key is promoted out of compaction

Context

The key does double duty: compaction’s winner selection (compaction_head, compaction_rank) and per-key delivery serialization (key_lease, taken when Concurrency = Defer). But the public API makes it reachable only through the compaction opt-in — ProduceOptions.Compaction *CompactionOptions{Key, Rank} — so a user cannot ask for strict per-key serialization while keeping every message: defer drags compaction’s supersede-and-drop semantics along. Kafka’s shape is the precedent: the key is a message property; what uses it (compaction, ordering) is policy on top. Surfaced while naming key_lease in [0611]: a lease named for compaction is a consume-side lock that has nothing to do with compaction.

Decision

Constructor choice superseded by 0691; other decisions are preserved there.

• The key becomes a message-level concept named message key.
• ProduceOptions gains a top-level MessageKey string; CompactionOptions keeps Rank, loses Key, and gains Enable bool — kept a pointer with an explicit flag for reading clarity (user-settled over one-mechanism-per-fact): nil and Enable false both mean not compacted. NewCompactionOptions(rank) sets Enable. Compaction enabled without MessageKey errors at produce time.
• common.MessageRow.CompactionKey → MessageKey, wire tag compaction_key → message_key. Column compaction_key → message_key in message_log and message_key_lease; compaction_head/compaction_rank keep their names — rank is rank within compaction.
• Concurrency = Defer requires only a message key, not compaction: serialized-by-key delivery with full history becomes expressible.

Consequences

• The produce hot path changes shape for every keyed caller; the read model and CLI --output json documents change key names. Pre-v1, no wire-compat machinery — docs, examples, and labs sweep in the same change (grep labs for ->>'compaction_key').
• Defer-without-compaction is a new behavior path: the key-lease claim logic must not assume a compaction head exists. Its labs (deferlab siblings) extend to the uncompacted case.
• Two same-key deferred rows can now share one exception batch (with compaction the older would be superseded); the loser at the key gate returns to status ‘deferred’ with its attempts increment decremented back, instead of sitting ‘inflight’ until lease expiry. message_log.compaction_rank becomes nullable — NULL is the row-level fact that a message never opted into compaction.
• Doc-site compaction pages rewrite around “the message key”, with compaction as one of its two uses; the produce page is the proposal surface before implementation (docs drive public-surface work).
• [0611]‘s message_key_lease rename lands with this change, never before it.