feat(audit): off-box chain-head export, verify-heads, maintenance login (ScadaBridge#107)

Item 1 (owner decision: no signing). AuditLog:HashChain:HeadExport (default
off): both centrals periodically read every month's chain head (LastSeq,
LastHash, purge anchor, chained rows, tombstones; one snapshot under the
chain lock SHARED) and append the changed heads to a sink the DB login cannot
write - File (append-only JSONL per node per day, local or UNC) or Syslog
(RFC 5424 over UDP / TCP octet counting). `audit verify-heads --path` reads
those records back (file or syslog-collector lines) and the new
VerifyAuditChainHeads command checks each against the database: Matches /
Extended / Purged, or Mismatch / Regressed / MissingHead / MissingLink /
CountMismatch (exit 3). AuditChainHeadStoreTests forges a chain with the
production hasher: verify-chain says Intact, verify-heads says Mismatch.

Item 3. AuditLog:MaintenanceConnectionString (${secret:}-capable, unset =
today's single login) runs the chain job, chain verify / head export, the
partition switch-out + archive and the per-channel purge on
AuditMaintenanceContext (keyed audit-maintenance repository/archive, purge
actor prefers them). AuditMaintenanceLoginTests runs the hardened setup on
SQL Server: writer (datareader/datawriter/audit_writer) ingests and is
refused chaining and deleting; the chainer+purger login does the full cycle.
Two-login grants documented in Component-AuditLog.md.

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

docs/requirements/Component-ManagementService.md

@@ -200,6 +200,7 @@ The two-person authorization workflow for writes through the MxAccess Gateway. B
 
 - **QueryAuditLog**: Legacy configuration-change audit query (filtered by entity type, user, date range, etc.) routed through `/management`. Gated to the `Admin` role; superseded for the centralized Audit Log component (#23) by the dedicated `/api/audit/*` REST endpoints described below.
 - **VerifyAuditChain** (ScadaBridge#41): verifies one UTC calendar month (`Month`, `YYYY-MM`) of the central AuditLog hash chain and returns an `AuditChainVerificationResult` (status, rows verified, tombstones applied, purge anchor, head sequence, unchained rows, first break with partition / `ChainSeq` / `EventId` / kind). Gated on the OperationalAudit roles (`Administrator`, `Viewer`); a site-scoped non-Administrator caller is refused (`ManagementUnauthorized`) because a month's chain spans every site. Long-running Ask timeout. Errors: `INVALID_ARGUMENT` (bad month), `CHAIN_BUSY` (a chaining/purge pass held the chain lock past 60 s). Backs `scadabridge audit verify-chain`.
+- **VerifyAuditChainHeads** (ScadaBridge#107): checks chain heads exported off-box (`Records`, at most 50,000 `AuditChainHeadRecord`s; duplicates collapsed) against the database chain and returns an `AuditChainHeadVerificationResult` (overall `Verified`, one `AuditChainHeadCheck` per distinct exported head: `Matches` / `Extended` / `Purged`, or the failures `Mismatch` / `Regressed` / `MissingHead` / `MissingLink` / `CountMismatch`). Same gate as VerifyAuditChain; long-running Ask timeout. Errors: `INVALID_ARGUMENT` (no records / over the cap), `CHAIN_BUSY`. Backs `scadabridge audit verify-heads`.
 - **ListAuditArchives** / **VerifyAuditArchive** (ScadaBridge#42): list the AuditLog Parquet cold archives under `AuditLog:Archive:Directory` (`AuditArchiveListResult`: enabled flag + every manifest), and verify one by `Name` (`AuditArchiveVerificationResult`: file integrity, per-month chain recomputed from the archived rows, anchor `Matches`/`Superseded`/`NotPurged`/`Mismatch`, overall `Verified`). Same gate as VerifyAuditChain (OperationalAudit roles, site-scoped non-Administrator refused). VerifyAuditArchive uses the long-running Ask timeout. Errors: `ARCHIVE_NOT_CONFIGURED`, `INVALID_ARGUMENT` (not an archive name), `NOT_FOUND`. Back `scadabridge audit archive list|verify`.
 
 ### Transport (Bundle Import / Export)