docs(changes): close §B4 with the S3 archive backend deferred by design

The operational break-glass ops (Owner provision, OP create, halt, Sync backup) are built and oracle-verified. The fourth §B4 line item — the tarS3 archive backend — is recorded as a deliberate deferral, not a silent gap: config.Load fail-closes store=tarS3 (frozen by TestLoadRejectsUnimplementedArchiveStore), tarLocal is the tested baseline every backup/restore path uses today, and offsite/cross-cluster DR is opt-in future work. Supersedes the "S3 remains open" note in the phase-2b peer doc.

docs/changes/INDEX.md

@@ -249,3 +249,4 @@ primary record.
 | 096d597 | 2026-07-07 | docs(changes): backfill detail docs for pre-ledger functional commits |
 | 5a7cd5a | 2026-07-07 | docs(changes): fold 346ec68 cloudflare-edge walkthrough into its detail doc |
 | 4626ab5 | 2026-07-07 | docs(changes): mutation-audit the ledger's "unit-tested" safety claims |
+| 729bd7b | 2026-07-07 | docs(changes): index the mutation audit and two lagging ledger rows |