feat(audit): hash-chain cutoff - chain only rows written from first enablement, no backlog (#139)

Owner decision on #107 (2026-11-13): enabling AuditLog:HashChain must not
chain the historical backlog. main chained every unchained row oldest-first.

- The first chaining pass on a pristine database (no head, no chained row)
  writes a chain-start anchor: AuditLogChainTombstone (cutoff month,
  ChainSeq 0), PurgedAtUtc = DB SYSUTCDATETIME(), RowHash =
  SHA-256("ScadaBridge.AuditLog.ChainCutoff.v1" || YYYY-MM || ticks).
  Written once under the exclusive chain lock, so both centrals agree
  whatever their clocks/config. No migration: existing table + grants.
- Only rows with OccurredAtUtc >= cutoff are chained. The anchor hash is
  the cutoff month's genesis, binding the cutoff into the chain.
- AuditLog:HashChain:ChainBacklog (default false) restores chain-everything,
  read only on the very first enablement. Legacy chains (heads, no anchor)
  keep chaining everything.
- verify-chain splits unchained rows into UnchainedRows vs PreCutoffRows
  (+ ChainFromUtc); new breaks ChainedBeforeCutoff (row re-dated before the
  cutoff, or cutoff moved forward) and CutoffAnchorInvalid (anchor hash /
  month / multiplicity). Archive verify uses the anchor from its own
  tombstone file or the DB.
- Partition switch-out keeps the anchor (deletes ChainSeq > 0 only); the
  head store / export / verify-heads tombstone count excludes it.
- CLI verify-chain table prints Chain from + Pre-cutoff rows.

Tests: AuditLogHashChainCutoffTests (7, MSSQL), 6 verifier unit tests,
service/CLI/management wire tests; 9 mutations all killed.

Claude-Session: https://claude.ai/code/session_01GT67ACmpMR7b9LGRPLijMi

tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/VerifyAuditChainCommandTests.cs

@@ -50,6 +50,27 @@ public class VerifyAuditChainCommandTests : TestKit
             new DateTime(2026, 5, 1, 0, 0, 0, DateTimeKind.Utc), Arg.Any<CancellationToken>());
     }
 
+    /// <summary>
+    /// ScadaBridge#139: the cutoff, the pre-cutoff row count and the new break kinds reach the
+    /// wire (the CLI renders them; the kinds as names like every other kind).
+    /// </summary>
+    [Fact]
+    public void CutoffFields_AndCutoffBreakKind_ReachTheWire()
+    {
+        var chainFrom = new DateTime(2026, 11, 13, 9, 30, 0, DateTimeKind.Utc);
+        var brk = new AuditChainBreak("2026-11", 4, Guid.NewGuid(), AuditChainBreakKind.ChainedBeforeCutoff, "moved back");
+        _chain.VerifyMonthAsync(Arg.Any<DateTime>(), Arg.Any<CancellationToken>())
+            .Returns(new AuditChainVerificationResult("2026-11", AuditChainStatus.Broken, 3, 0, null, 9, 0, brk, 3441, chainFrom));
+
+        CreateActor().Tell(Envelope(new VerifyAuditChainCommand("2026-11"), [], "Viewer"));
+
+        var response = ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+        using var json = JsonDocument.Parse(response.JsonData);
+        Assert.Equal(3441, json.RootElement.GetProperty("preCutoffRows").GetInt64());
+        Assert.Equal(chainFrom, json.RootElement.GetProperty("chainFromUtc").GetDateTime().ToUniversalTime());
+        Assert.Equal("ChainedBeforeCutoff", json.RootElement.GetProperty("firstBreak").GetProperty("kind").GetString());
+    }
+
     /// <summary>A Designer/Deployer without an OperationalAudit role is refused.</summary>
     [Fact]
     public void NonAuditRole_IsUnauthorized()