refactor: extract audit surface into DomainViewModel+Audit (v5 step 4, 1/n)

First slice of the v5.0.0 god-file decomposition. DomainViewModel.swift is a
single ~4.9k-line class body; this begins splitting it into focused
`DomainViewModel+<Concern>.swift` extensions, one cohesive concern at a time,
with each move behavior-preserving and verified by build + the test net.

- Moves the audit read/CRUD/export surface (audits/auditSession/auditTimeline,
  updateAuditStatus/Notes, toggleAuditChecklistItem, add/update/remove
  AuditFinding, exportAuditData) into DomainViewModel+Audit.swift as an
  extension. Pure move — no logic changes.
- `startAudit(for:)` intentionally stays on the main type: it drives a live
  inspection to seed the session, so it belongs with the inspection pipeline
  until that is extracted. `persistAuditSessions()` is promoted from private to
  internal so both files can call it (its only cross-file dependency).

No project.pbxproj change is needed — DomainDig/ is a file-system-synchronized
group, so the new file is picked up automatically.

DomainViewModel.swift: 4864 -> 4761 lines. App builds clean; unit suite 58/58.

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