feat(audit): emit events for export, purge, reindex

[bomanaps]

Summary

• Wires BALP-compliant AuditEvent emission for $purge, $reindex, and bulk
  export — closing Persistence-layer audit events for bulk export, purge, and reindex #168.
• $purge and $reindex land as new REST handlers backed by an OperationsBundle,
  while the bulk-export worker now emits worker-complete / cancelled / failed events
  at the actual lifecycle boundary instead of leaving them silent.
• Thirteen integration tests assert action codes, outcomes, and detail fields
  end-to-end via a new InMemoryAuditSink test fixture.

Test plan

• cargo fmt --all -- --check
• cargo clippy --no-deps on helios-audit, helios-rest,
  helios-persistence, helios-hfs
• cargo test -p helios-audit -p helios-rest -p helios-persistence -p helios-hfs (1623 pass, 0 fail)
• Manual end-to-end run against the hfs binary with HFS_AUDIT_BACKEND=file,
  confirming 11 expected AuditEvent records emit across $purge, $reindex, and the
  full bulk-export lifecycle (including worker-emitted completion)