docs(audit): Parquet cold archive shipped, default off (ScadaBridge#42)

Component-AuditLog Retention & Purge describes the archive: directory and
layout, the three files, consistency with the hash chain (SHARED read,
guarded switch, anchor = archived chain end), fail-closed purge, retention
interplay (per-channel overrides are not archived, their tombstones are),
verify/read, and the AuditLog:Archive configuration. alog.md §13.6/§15.2/§16,
CLAUDE.md, HighLevelReqs AL-12, Component-CLI, Component-ManagementService,
docs/components/{CLI,ManagementService}.md and the CLI README drop the
"deferred / 501" wording and document `audit archive list|verify` and the
parquet export.

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

CLAUDE.md

@@ -181,7 +181,8 @@ spec for each is `docs/requirements/Component-<Name>.md`, and `README.md` carrie
 - Payload cap 8 KB by default / 64 KB on error rows; auth headers redacted by default; SQL parameter values captured by default; per-target redaction opt-in. Inbound API: full verbatim capture up to `InboundMaxBytes` (default 1 MiB); request headers stored in `Extra.requestHeaders` (post-redaction); per-method `SkipBodyCapture` flag suppresses bodies while still recording headers + metadata; `AuditInboundCeilingHits` counter surfaced on health snapshot. (M5.3 T7)
 - Audit-write failure NEVER aborts the user-facing action — audit is best-effort, the action's own success/failure path is authoritative.
 - 365-day central retention with monthly partition-switch purge; per-channel retention overrides (`AuditLog:PerChannelRetentionDays`) expire rows earlier than the global window via a bounded, batched row DELETE on the purge actor's maintenance path — values must be shorter than the global window (M5.5 T3); 7-day site SQLite retention with a hard `ForwardState` invariant (no row purged until forwarded or reconciled).
-- Append-only enforced via DB roles (writer role has INSERT only, no UPDATE/DELETE); Parquet archival (T2) is deferred to v1.x.
+- Append-only enforced via DB roles (writer role has INSERT only, no UPDATE/DELETE).
+- **Parquet cold archive (T2, ScadaBridge#42) — default OFF (`AuditLog:Archive:Enabled` + absolute `Directory`, shared by both centrals).** The purge actor writes each expired partition to `{Directory}/{YYYY}/auditlog-{YYYY-MM}-{stamp}Z.parquet` (10 stored columns + `ChainSeq`/`RowHash`, tick-exact `OccurredAtUtcTicks`) plus `.tombstones.parquet` and a `.manifest.json` (written last; chain head at archive time, SHA-256 per file), read under the chain lock SHARED; then the switch runs with a guard (under the EXCLUSIVE lock: partition row count + each month's `LastSeq` must still equal the archive's, else THROW 51002/51003 and nothing is purged). Fail closed: a partition that cannot be archived is not purged. `audit archive list|verify` = `ListAuditArchives`/`VerifyAuditArchive` (verify-chain's gate): file hashes, chain recomputed from the archive, chain end vs the DB purge anchor (`Matches`/`Superseded`/`NotPurged`/`Mismatch`), CLI exit 3. `audit export --format parquet` streams the 24 export columns (was 501). No migration; no re-import path (open decision), object storage not wired (mount it).
 - **Hash-chain tamper evidence (T1, ScadaBridge#41) — default OFF (`AuditLog:HashChain:Enabled`).** One SHA-256 chain per UTC calendar month: `ChainSeq` + `RowHash = SHA-256(prev || canonical(row))` on `AuditLog` (migration `AddAuditLogHashChain`), linked by the central `AuditLogHashChainService` (both nodes, serialised by `sp_getapplock`) — never by the ingest path. `AuditLogChainHead` catches tail deletion and carries the partition-switch checkpoint anchor; the per-channel DELETE writes an `AuditLogChainTombstone` per chained row in the same transaction. `audit verify-chain --month` = management command `VerifyAuditChain` (OperationalAudit, unscoped callers only), first break with partition/ChainSeq/EventId, CLI exit 3. A chained row's `SourceNode` is frozen (backfill skips it). Unsigned; signing/external anchoring are open.
 - Node-of-origin is captured alongside site-of-origin: `SourceNode` (`varchar(64)` NULL) on `AuditLog`, `Notifications`, and `SiteCalls` — `node-a`/`node-b` for site rows (qualified by `SourceSiteId`/`SourceSite`), `central-a`/`central-b` for central direct-write rows. Stamped at the writing node, carried verbatim through telemetry + reconciliation, and indexed via `IX_AuditLog_Node_Occurred (SourceNode, OccurredAtUtc)` on `AuditLog`.
 - Per-node stuck KPIs (M5.3 T6): Notification Outbox and Site Call Audit expose `PerNodeNotificationKpiRequest`/`PerNodeSiteCallKpiRequest` messages that group stuck/parked/delivered counts by `SourceNode`, surfacing per-node breakdowns on the Health dashboard.