refactor(deploy): evaluator groups instances by site once; doc corrections

Tallying filtered the whole deployed-instance set inside the site loop, making the
aggregate O(sites x instances) — the one per-entity loop in an otherwise batched
pass. Group once into a lookup and index per site. Also: ResolveArtifactState's
missing-hash branch is a defensive default, not the race the comment described
(the same site list feeds the hash computation), and the staleness batch omits an
instance undeployed mid-batch as well as one that never had a snapshot.

Claude-Session: https://claude.ai/code/session_01Y5ef34iUUMjmwnMpL9xgmB

src/ZB.MOM.WW.ScadaBridge.DeploymentManager/PendingUpdates/PendingUpdatesEvaluator.cs

@@ -78,10 +78,15 @@ public sealed class PendingUpdatesEvaluator : IPendingUpdatesEvaluator
         var staleness = await _deploymentService.GetInstanceStalenessAsync(
             deployedInstances.Select(i => i.Id).ToList(), cancellationToken);
 
+        // Grouped ONCE, not re-scanned per site: tallying by filtering the whole
+        // deployed set inside the site loop would make the aggregate O(sites x
+        // instances) — the one genuinely per-entity loop in an otherwise batched pass.
+        var instancesBySite = deployedInstances.ToLookup(i => i.SiteId);
+
         var rows = new List<SitePendingUpdates>(sites.Count);
         foreach (var site in sites.OrderBy(s => s.Name, StringComparer.OrdinalIgnoreCase))
         {
-            var (deployed, stale, unknown) = TallyInstances(site.Id, deployedInstances, staleness);
+            var (deployed, stale, unknown) = TallyInstances(instancesBySite[site.Id], staleness);
             rows.Add(new SitePendingUpdates(
                 site.Id,
                 site.SiteIdentifier,
@@ -114,9 +119,16 @@ public sealed class PendingUpdatesEvaluator : IPendingUpdatesEvaluator
     /// Decides a site's artifact state by comparing the hash stamped by its last
     /// successful artifact deploy against the hash the current definitions would
     /// produce. A site with no stamp has never had artifacts deployed, which is
-    /// <see cref="ArtifactDeploymentState.NeverDeployed"/>, NOT out of date. A site
-    /// missing from the computed map (it appeared between the two reads) is reported
-    /// out of date, the side that prompts an operator to look rather than hiding it.
+    /// <see cref="ArtifactDeploymentState.NeverDeployed"/>, NOT out of date.
+    ///
+    /// <para>
+    /// A stamped site missing from the computed map is reported out of date. That is a
+    /// defensive default, not a reachable race: this call path hands the SAME site list
+    /// to <see cref="ArtifactDeploymentService.ComputeCurrentArtifactHashesAsync"/>,
+    /// which sets an entry for every input site or throws. It is kept so that a future
+    /// producer of a partial map fails toward VISIBLE (out of date, an operator looks)
+    /// rather than hidden (current, a stale site silently reads clean).
+    /// </para>
     /// </summary>
     /// <param name="site">The site to classify.</param>
     /// <param name="currentArtifactHashes">Site id to the hash a deploy would send right now.</param>
@@ -137,26 +149,25 @@ public sealed class PendingUpdatesEvaluator : IPendingUpdatesEvaluator
     /// <summary>
     /// Counts one site's deployed, stale and un-evaluatable instances. Only instances
     /// present in the staleness result are counted — the batch OMITS an instance with
-    /// no deployed snapshot, which is exactly the "nothing to be stale against"
-    /// exclusion the counts need.
+    /// no deployed snapshot at evaluation time (including one that was undeployed
+    /// mid-batch), which is exactly the "nothing to be stale against" exclusion the
+    /// counts need.
     /// </summary>
-    /// <param name="siteId">Site whose instances are being tallied.</param>
-    /// <param name="deployedInstances">All instances in a state other than NotDeployed.</param>
+    /// <param name="siteInstances">
+    /// This site's instances in a state other than NotDeployed, pre-grouped by the caller.
+    /// </param>
     /// <param name="staleness">Instance id to stale (true) / current (false) / unknown (null).</param>
     /// <returns>The deployed, stale and unknown counts for the site.</returns>
     private static (int Deployed, int Stale, int Unknown) TallyInstances(
-        int siteId,
-        IReadOnlyList<Instance> deployedInstances,
+        IEnumerable<Instance> siteInstances,
         IReadOnlyDictionary<int, bool?> staleness)
     {
         var deployed = 0;
         var stale = 0;
         var unknown = 0;
 
-        foreach (var instance in deployedInstances)
+        foreach (var instance in siteInstances)
         {
-            if (instance.SiteId != siteId)
-                continue;
             if (!staleness.TryGetValue(instance.Id, out var isStale))
                 continue;