0530 — pkg/compaction stays two-layer; MessageRow lives in common

Context

Every other domain follows pkg/ -> pkg//controller -> pkg//controller/datastore. Compaction has no pkg/compaction vocabulary package; its read-model is common.MessageRow. The domain is also co-owned: the head upsert and the FOR UPDATE head read inside a produce transaction belong to the producer stack, while pkg/compaction/controller is the plain read door. Producer (GetCompactionHeadInTx, aliased through producer/controller), alert’s classify, and the compaction controller all return or take MessageRow.

Decision

• Compaction stays two-layer as a deliberate exception: it owns no vocabulary, so no pkg/compaction package exists to hold none.
• MessageRow stays in pkg/common. It is shared across the producer and compaction stacks, and common is the sanctioned home for vocabulary shared across different stacks (same rule that places shared error sentinels there).
• Moving MessageRow into a new pkg/compaction was rejected: producer would import another domain’s vocabulary for a row the producer itself writes, and the package would exist to hold exactly one moved struct.

Consequences

• The three-layer template stays the default; compaction is the one recorded exception, revisited only if compaction grows vocabulary of its own (sentinels, enums, a second read-model).
• Callers keep common.MessageRow (or the producer alias) in signatures; no import changes anywhere.