Phase 892: RPC-first fallback helper + response validators split (v0.8.859)

Non-breaking refactor — scheduleGuildGossipRpcFirstWithFallback() dedupes RPC-first
fire-and-forget paths; gossip-rpc-response-validators.js splits isValid* predicates;
search/audit/export mixins migrate to centralized isGuildGossipRpcRequestEnabled gate.
test:ci-phase892 passes (46/50, cutover deferred).

mixins/runtime/guild-sync/platform-guild-sync-export-mixin.js

@@ -28,6 +28,9 @@ const {
   id, now, guildTopic
 } = require('../../../platform-class-imports')
 const { platformGuildSyncRpcRespondersMixin } = require('./platform-guild-sync-rpc-responders-mixin')
+const {
+  scheduleGuildGossipRpcFirstWithFallback
+} = require('../../../gossip-rpc-rpc-first-fallback')
 
 
 /** Phase 756 — guild sync bundle export/push */
@@ -306,12 +309,10 @@ requestGuildMemberPage (offset = 0, opts = {}) {
       })
       .catch(() => {})
   }
-  if (sharedScope.isRpcGossipRequestEnabled?.()) {
-    void this._tryMemberPageRequestRpc(off)
-      .then((rpc) => {
-        if (!rpc?.viaRpc) fanoutMemberPageRequest()
-      })
-      .catch(() => fanoutMemberPageRequest())
+  if (scheduleGuildGossipRpcFirstWithFallback(
+    () => this._tryMemberPageRequestRpc(off),
+    fanoutMemberPageRequest
+  )) {
     return {
       guildId,
       offset: off,