fix(opcua): #855 perf review 3 — weak filter-group registry; where clause validated; sharing pinned

- The filter-group registry holds groups weakly (the filter->group table is
  the only strong reference), sweeps dead entries amortised and counts only
  live shapes against the 4096 cap; filters past the cap are counted on the
  new meter otopcua.opcua.condition_events.filter_groups_unshared.
- A group validates its where clause too (restored, never-validated filters
  evaluate like validated ones).
- Test: value-equal filters cost one projection per locale set per report.

Claude-Session: https://claude.ai/code/session_01GT67AD99zQQqpLCVm3p7xk

src/Core/ZB.MOM.WW.OtOpcUa.Commons/Observability/OtOpcUaTelemetry.cs

@@ -177,6 +177,15 @@ public static class OtOpcUaTelemetry
     ///     item had already replayed a newer state (Gitea #854) — one per item and event. Every other subscriber still
     ///     receives them; this only stops a refresh being overtaken by older events.
     /// </summary>
+    /// <summary>
+    ///     Event filters given a private projection group because the server-wide registry of distinct live filter shapes is
+    ///     full (Gitea #855 perf). Delivery is unchanged; only the per-report sharing of where-clause / field projection
+    ///     stops for those filters, so a steadily rising count means more per-item work under the node-manager lock.
+    /// </summary>
+    public static readonly Counter<long> ConditionEventFilterGroupsUnshared =
+        Meter.CreateCounter<long>("otopcua.opcua.condition_events.filter_groups_unshared", unit: "{filter}",
+            description: "Event filters not shared because the registry of live filter shapes is full.");
+
     public static readonly Counter<long> ConditionEventDeliveriesSuperseded =
         Meter.CreateCounter<long>("otopcua.opcua.condition_events.superseded", unit: "{delivery}",
             description: "Deferred condition events withheld from a Server-object item whose ConditionRefresh already replayed a newer state.");