chore(followups): reviewer-recommended tests, comments, and hardening from the remediation reviews

The remediation reviews approved every task but left a tail of small notes.
This lands the gateway-side half of them.

Hardening (behavior changes, all narrow):

- BuildFilteredWriteBulkCommand's unreachable default case failed OPEN: a
  fifth bulk-write kind added upstream without a filter case here would have
  shipped the DENIED entries to the worker while reporting them denied to the
  caller. It now throws UnreachableException.
- SqliteCanonicalAuditStore.ListRecentAsync no longer throws on a row it
  cannot date. The retention sweep deliberately preserves such rows (SQLite's
  datetime() yields NULL, so the DELETE never matches), which guaranteed the
  dashboard's recent-audit view would meet one eventually and lose the whole
  page to it. The row is now reported at DateTimeOffset.MinValue with every
  other column intact, behind an optional logger.
- The audit drain loop's finally now completes the channel writer alongside
  detaching the drain, so a producer that raced past the attached check takes
  the write-through branch instead of stranding its event in a buffer nobody
  reads until shutdown. TryComplete is idempotent, so StopAsync is unaffected.

Tests:

- MapCommandReply ownership (Assert.Same on the inner reply), mirroring the
  existing MapEvent ownership test.
- Redactor key-id length boundary at exactly 64 and 65 characters, pinning
  which way it fails. Nothing validates key-id length at creation, so
  docs/Diagnostics.md's "which no issued key id does" is now stated as the
  heuristic it is.
- ApiKeyFailureLimiter.Reset with a PartitionResolution whose partition was
  evicted between the Check and the Reset: inert, and clears nobody else's
  block.
- Constraint-cache concurrency stress: the cap is enforced by the inserting
  thread, so overshoot must be transient and proportional to the in-flight
  inserters, and the cache must settle at or under the cap.
- ListRecentAsync against a raw-SQL undateable row.

Comment/doc accuracy:

- EventsHubViewerRegistry.ReleaseConnection records that it relies on
  SignalR's default sequential per-connection dispatch
  (MaximumParallelInvocationsPerClient = 1).
- A PERF(followup) note on Invoke's double session resolve and why removing it
  needs a SessionManager overload.
- SessionEventDistributor: the volatile-field comment named the pump as the
  lock-free reader, but the pump's single capture point is inside _replayLock;
  the genuinely lock-free reader is SubscriberCount. OnSubscriberOverflow's
  "cannot be observed here" now excepts the DisposeAsync abandon path. The
  churn test names its ConcurrentDictionary bucket-order assumption and that a
  violation surfaces as a read timeout, not a silent pass.
- The two "restores the sequential drain's behavior" claims (SessionManager,
  docs/Sessions.md) were wrong: the sequential drain leaked too, because
  KillWorkerAsync's entry ThrowIfCancellationRequested aborted the whole loop
  on the first session for zero kills. Reworded to "fixes a leak the
  sequential drain also had", with the sweep-bound/shutdown-unbound
  ParallelOptions asymmetry explained.
- ISessionManager.ShutdownAsync's token doc: it degrades the drain to a kill
  sweep rather than cancelling it, with the bounded overrun stated.
  SessionShutdownHostedService.StopAsync records that its cancellation-logging
  branch is now unreachable.

src/ZB.MOM.WW.MxGateway.Tests/Security/Authentication/CachingApiKeyVerifierTests.cs

@@ -281,6 +281,55 @@ public sealed class CachingApiKeyVerifierTests
         Assert.Same(reparsed, MapConstraints(firstJson));
     }
 
+    /// <summary>
+    ///     Stress: the constraint cache's bound is enforced by the inserting thread itself
+    ///     (<c>GetOrAdd</c>, then enqueue, then evict), so concurrent inserters can each land an
+    ///     entry before any of them reaches the eviction step. That overshoot is real but must be
+    ///     transient and proportional to the in-flight inserters — not unbounded growth — and once
+    ///     the churn stops the cache must be back at or under the cap. Hammered on both the
+    ///     converging path (every iteration maps the same blob, so all but one <c>GetOrAdd</c>
+    ///     loses) and the growth path (a distinct blob per iteration, which is what forces
+    ///     eviction).
+    /// </summary>
+    [Fact]
+    public void ToGatewayIdentity_ConcurrentBlobs_OvershootIsTransientAndCacheSettlesUnderCap()
+    {
+        const int cap = GatewayApiKeyIdentityMapper.MaxCachedConstraintBlobs;
+
+        // Overshoot is bounded by how many inserters can sit between their GetOrAdd and their own
+        // EvictIfOverCapacity, so scale the allowance with the available parallelism rather than
+        // pinning a magic number. Generous on purpose: the assertion under test is "bounded", not
+        // "bounded by exactly this".
+        int transientAllowance = (Environment.ProcessorCount * 8) + 64;
+        string sharedJson = ConstraintsJson("Area_StressShared");
+        int peak = 0;
+
+        Parallel.For(0, (cap * 2) + 64, index =>
+        {
+            MapConstraints(sharedJson);
+            MapConstraints(ConstraintsJson($"Area_Stress_{index}"));
+
+            int size = GatewayApiKeyIdentityMapper.CurrentCacheSize;
+            int seen = Volatile.Read(ref peak);
+            while (size > seen && Interlocked.CompareExchange(ref peak, size, seen) != seen)
+            {
+                seen = Volatile.Read(ref peak);
+            }
+        });
+
+        Assert.True(
+            peak <= cap + transientAllowance,
+            $"cache peaked at {peak} entries, past the {cap} cap plus the {transientAllowance} transient allowance");
+
+        // The cache is process-wide static and other test classes in this assembly map identities
+        // too, so poll rather than asserting on the instant the loop returns.
+        Assert.True(
+            SpinWait.SpinUntil(
+                () => GatewayApiKeyIdentityMapper.CurrentCacheSize <= cap,
+                TimeSpan.FromSeconds(5)),
+            $"cache settled at {GatewayApiKeyIdentityMapper.CurrentCacheSize} entries, past the {cap} cap");
+    }
+
     private static ApiKeyConstraints MapConstraints(string constraintsJson) =>
         GatewayApiKeyIdentityMapper.ToGatewayIdentity(new LibApiKeyIdentity(
             KeyId: "operator01",