Refactor SurfaceCacheValidator to simplify oldest entry calculation

Add global using for Xunit in test project

Enhance ImportValidatorTests with async validation and quarantine checks

Implement FileSystemQuarantineServiceTests for quarantine functionality

Add integration tests for ImportValidator to check monotonicity

Create BundleVersionTests to validate version parsing and comparison logic

Implement VersionMonotonicityCheckerTests for monotonicity checks and activation logic

docs/airgap/runbooks/quarantine-investigation.md

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+# AirGap Quarantine Investigation Runbook
+
+## Purpose
+Quarantine preserves failed bundle imports for offline forensic analysis. It keeps the original bundle and the verification context (reason + logs) so operators can diagnose tampering, trust-root drift, or packaging issues without re-running in an online environment.
+
+## Location & Structure
+Default root: `/updates/quarantine`
+
+Per-tenant layout:
+`/updates/quarantine/<tenantId>/<timestamp>-<reason>-<id>/`
+
+Removal staging:
+`/updates/quarantine/<tenantId>/.removed/<quarantineId>/`
+
+## Files in a quarantine entry
+- `bundle.tar.zst` - the original bundle as provided
+- `manifest.json` - bundle manifest (when available)
+- `verification.log` - validation step output (TUF/DSSE/Merkle/rotation/monotonicity, etc.)
+- `failure-reason.txt` - human-readable failure summary (reason + timestamp + metadata)
+- `quarantine.json` - structured metadata for listing/automation
+
+## Investigation steps (offline)
+1. Identify the tenant and locate the quarantine root on the importer host.
+2. Pick the newest quarantine entry for the tenant (timestamp prefix).
+3. Read `failure-reason.txt` first to capture the top-level reason and metadata.
+4. Review `verification.log` for the precise failing step.
+5. If needed, extract and inspect `bundle.tar.zst` in an isolated workspace (no network).
+6. Decide whether the entry should be retained (for audit) or removed after investigation.
+
+## Removal & Retention
+- Removal requires a human-provided reason (audit trail). Implementations should use the quarantine service’s remove operation which moves entries under `.removed/`.
+- Retention and quota controls are configured via `AirGap:Quarantine` settings (root, TTL, max size); TTL cleanup can remove entries older than the retention period.
+
+## Common failure categories
+- `tuf:*` - invalid/expired metadata or snapshot hash mismatch
+- `dsse:*` - signature invalid or trust root mismatch
+- `merkle-*` - payload entry set invalid or empty
+- `rotation:*` - root rotation policy failure (dual approval, no-op rotation, etc.)
+- `version-non-monotonic:*` - rollback prevention triggered (force activation requires a justification)