feat(audit): Parquet cold archive of expired AuditLog partitions before the purge (ScadaBridge#42)

Audit roadmap T2 (alog.md §15.2): with AuditLog:Archive:Enabled (default off)
the purge actor writes each expired monthly partition to Parquet under an
absolute AuditLog:Archive:Directory before switching it out, so audit rows
outlive RetentionDays.

- Parquet.Net 6.1.0 (fully managed; NuGetAudit clean) in central package
  management.
- AuditArchiveParquet: data file = the ten stored columns, OccurredAtUtc as a
  UTC microsecond timestamp plus exact OccurredAtUtcTicks, and ChainSeq/RowHash;
  tombstone file = the month's per-channel purge tombstones. Buffered row
  groups; the files are plain Parquet (DuckDB/pyarrow read them).
- AuditLogPartitionArchive (IAuditLogArchive): reads the partition's rows,
  chain heads and tombstones in one transaction under the chain lock SHARED,
  writes {Directory}/{YYYY}/auditlog-{YYYY-MM}-{stamp}Z.parquet,
  .tombstones.parquet and a .manifest.json (counts, SHA-256 per file, each
  month's head and prior anchor) via tmp + rename, manifest last. Archives are
  never overwritten.
- Archive before purge: SwitchOutPartitionAsync gains an internal guarded
  overload. Under the EXCLUSIVE chain lock it re-counts the partition and
  re-reads each month's LastSeq and THROWs 51002/51003 (nothing purged) unless
  both still equal the archive's, so the switch's checkpoint anchor is exactly
  the archive's chain end. The manifest is then marked Purged.
- Fail closed: a partition that cannot be archived is not purged; the failure
  takes the existing AuditLogPurgeFailedEvent / PurgeFailures path. Success
  publishes AuditLogArchivedEvent.
- Verify: file hashes and counts against the manifest, the chain recomputed
  from the archived rows and tombstones (AuditChainVerifier), and the chain end
  against the database anchor (Matches / Superseded / NotPurged / Mismatch).
- AuditLogArchiveOptionsValidator: Directory required when enabled and must be
  absolute; RowGroupSize, timeouts bounded.

No migration: the purger role already reads AuditLog, the head and tombstones.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/ZB.MOM.WW.ScadaBridge.ConfigurationDatabase/ServiceCollectionExtensions.cs

@@ -114,6 +114,15 @@ public static class ServiceCollectionExtensions
         // AuditLogHashChainService, verified by the VerifyAuditChain management command.
         services.AddScoped<IAuditLogHashChain, AuditLogHashChain>();
 
+        // AuditLog Parquet cold archive (ScadaBridge#42): written by the purge actor before a
+        // partition switch, listed/verified by the ListAuditArchives / VerifyAuditArchive
+        // management commands. AuditLog:Archive is bound by AddAuditLogCentral; without it the
+        // defaults apply (disabled, no directory).
+        services.AddScoped<IAuditLogArchive>(sp => new AuditLogPartitionArchive(
+            sp.GetRequiredService<ScadaBridgeDbContext>(),
+            sp.GetService<Microsoft.Extensions.Options.IOptions<AuditLogArchiveOptions>>()?.Value ?? new AuditLogArchiveOptions(),
+            sp.GetService<Microsoft.Extensions.Logging.ILogger<AuditLogPartitionArchive>>()));
+
         // Opt-in config-audit retention (ScadaBridge:ConfigAudit:RetentionDays, default 0 = off).
         // Options binding to configuration is done by the host; without it the defaults apply
         // and the purge service is a no-op.