bug(codex): a live compaction can show "Conversation compacted" twice #1401

Found by review c of #1386.

Sequence (reproduced through the real ledger)

1. The live block. Codex compacts in a live, proxy-observed session. The app-server compaction response item becomes a finalized semantic block (CodexResponsesAdapter.mapItemTypeToBlockKind, case 'compaction').
2. The durable boundary. The CLI then writes the compacted rollout line, which fix(codex): render committed compaction as a timestamped boundary #1386 maps to a timestamped system:compact_boundary.
3. Both rows survive. collectSemanticCandidates admits the block as assistant-text with no text keys, and buildCommittedOwnership does not index compact-boundary against it, so decideLiveCandidate suppresses neither.
4. Result. Both render through shared.compaction as a muted "Conversation compacted" row.

Scope

• Cosmetic and transient. After a reload the semantic history is empty, so only the boundary remains.
• Not observed in the corpus. No frozen bundle contains both surfaces; each channel has captured real compactions separately.
• Claude already suppresses this class, through isCompactionSynthesis turns being rejected in observations/semantic.ts. Codex's semantic item has no such flag.

Direction

Suppress the finalized semantic compaction row once a durable boundary is committed whose timestamp is at or after the block's turn end. Alternatively, give the two different labels.