feat(dashboard): GroupToTag / UntaggedSessionVisibility config (SEC-25)

Groundwork for the per-session dashboard event ACL (docs/plans/2026-07-10-dashboard-session-acl-tst15.md 3.2): a dashboard group can now grant visibility tags, and untagged sessions default to AdminOnly. Enforcement lands with the EventsHub ACL; nothing consumes the grant yet.

GroupToTag is deliberately uncoupled from GroupToRole - a group may appear in either map, both, or neither - and is validated for shape only. Tags gate dashboard event visibility, never data access.

src/ZB.MOM.WW.MxGateway.Server/Configuration/DashboardOptions.cs

@@ -67,4 +67,20 @@ public sealed class DashboardOptions
     /// Users with no matching group are rejected at login.
     /// </summary>
     public Dictionary<string, string> GroupToRole { get; init; } = new(StringComparer.OrdinalIgnoreCase);
+
+    /// <summary>
+    /// LDAP group → dashboard visibility tags. A dashboard user's granted tag set
+    /// is the union over the groups they belong to; a session is observable on the
+    /// events hub when its tags intersect that grant. Independent of
+    /// <see cref="GroupToRole"/> — a group may appear in either map, both, or
+    /// neither. Visibility only: tags never gate data access.
+    /// </summary>
+    public Dictionary<string, string[]> GroupToTag { get; init; } = new(StringComparer.OrdinalIgnoreCase);
+
+    /// <summary>
+    /// Who may observe a session whose owning API key carries no dashboard tags.
+    /// Defaults to <see cref="Configuration.UntaggedSessionVisibility.AdminOnly"/>
+    /// so an upgrade tightens rather than loosens.
+    /// </summary>
+    public UntaggedSessionVisibility UntaggedSessionVisibility { get; init; } = UntaggedSessionVisibility.AdminOnly;
 }

src/ZB.MOM.WW.MxGateway.Server/Configuration/GatewayOptionsValidator.cs

@@ -410,6 +410,36 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
             }
         }
 
+        // GroupToTag is validated for shape only, and independently of GroupToRole:
+        // a group may grant a role, a tag, both, or neither. An empty map is legal —
+        // it yields Viewers with no tag grant, which (under the default AdminOnly)
+        // means they observe no session's events. That is the fail-closed posture.
+        foreach (KeyValuePair<string, string[]> entry in options.GroupToTag)
+        {
+            if (string.IsNullOrWhiteSpace(entry.Key))
+            {
+                builder.Add("MxGateway:Dashboard:GroupToTag keys (LDAP group names) must be non-blank.");
+            }
+
+            if (entry.Value is null)
+            {
+                builder.Add($"MxGateway:Dashboard:GroupToTag['{entry.Key}'] must be a list of tags, not null.");
+                continue;
+            }
+
+            if (Array.Exists(entry.Value, string.IsNullOrWhiteSpace))
+            {
+                builder.Add($"MxGateway:Dashboard:GroupToTag['{entry.Key}'] tags must be non-blank.");
+            }
+        }
+
+        if (!Enum.IsDefined(options.UntaggedSessionVisibility))
+        {
+            builder.Add(
+                $"MxGateway:Dashboard:UntaggedSessionVisibility must be '{nameof(UntaggedSessionVisibility.AdminOnly)}' "
+                + $"or '{nameof(UntaggedSessionVisibility.AllViewers)}'.");
+        }
+
         AddIfNotPositive(
             options.SnapshotIntervalMilliseconds,
             "MxGateway:Dashboard:SnapshotIntervalMilliseconds must be greater than zero.",

src/ZB.MOM.WW.MxGateway.Server/Configuration/UntaggedSessionVisibility.cs

@@ -0,0 +1,22 @@
+namespace ZB.MOM.WW.MxGateway.Server.Configuration;
+
+/// <summary>
+/// Who may observe the dashboard event stream of a session that carries no
+/// dashboard tags. Tags gate dashboard event VISIBILITY only; they never widen
+/// or narrow data access.
+/// </summary>
+public enum UntaggedSessionVisibility
+{
+    /// <summary>
+    /// Default. An untagged session is visible only to a dashboard Administrator.
+    /// Fails closed: a deployment that has not populated
+    /// <see cref="DashboardOptions.GroupToTag"/> shows Viewers nothing.
+    /// </summary>
+    AdminOnly,
+
+    /// <summary>
+    /// An untagged session is visible to every dashboard Viewer. Opt-in for a
+    /// genuinely single-tenant deployment that wants the pre-ACL behaviour.
+    /// </summary>
+    AllViewers
+}