CultNet: refuse hop edges whose citer is a variant; withdraw a refused hop subscription

An edge carries the reference payload of its citer's resolved document, and the
citer is never a selected row under `cited`, so a variant citer slipped past the
selected-row refusal. Edges are now checked by the same rule (typed
NotSupportedException naming the variant), on the paged and the whole-set paths.

Reconcile refused a hop subscriber whose selection selected a variant without
withdrawing it, so it was told again on every store change. It withdraws as the
subscribe path does.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/GameCult.Networking/CultNetDatabaseSubscriptionServer.cs

@@ -358,8 +358,10 @@ namespace GameCult.Networking
             }
             catch (NotSupportedException refusal)
             {
-                // Q6: the selection selects a variant, which CultNet cannot carry. This subscriber is told, by key, and keeps what
-                // it was delivered; a change never throws out of the change handler.
+                // Q6: the selection selects a variant, which CultNet cannot carry. Refused as the subscribe path refuses it: the
+                // subscription is withdrawn, so the peer is told once, not once per store change; a change never throws out of
+                // the change handler.
+                Withdraw(key, sendRemovals: false, forgetRequest: true);
                 key.Peer.SendCultNet(new CultNetErrorMessage { Error = refusal.Message });
                 return;
             }