feat: P0-1e expand handleConfigPush with host config and cluster dispatch

Wires the remaining cluster_config_t data into the notification pipeline:
- dispatchConfigPush() extracted as a pure-Go helper (CGO-free, testable)
  that drives OnQueueUpdated, OnResourceUpdated, OnHostConfigUpdated, and
  OnClusterUpdated from a single config push event
- handleConfigPush() delegates to dispatchConfigPush via the new helper,
  eliminating the previous inline loops
- OnHostConfigUpdated and OnClusterUpdated added to NotificationHandler
  interface and implemented with partial-merge semantics in DefaultHandler
- rawHostConfigData/rawHostConfigToModel and rawClusterData/rawClusterToModel
  added to convert_pure.go with their CGO extraction counterparts in convert.go

Safety fixes from code review:
- OnHostUpdated, OnHostConfigUpdated, OnClusterUpdated now copy *incoming to
  a local merged value before mutation, closing the TOCTOU window between
  store.Get and store.Put under concurrent pushes
- cClusterConfQueues and cClusterConfResources gain nil-pointer guards
  matching the existing guard in cClusterConfHostConfigs
- handleConfigPush gains an explicit gHandler nil check
- cClusterConfToClusterInfo returns nil on empty clustername (prevents phantom
  empty-key write to store.Clusters); handleConfigPush logs a warning when nil
- 36 unit tests passing (CGO_ENABLED=0)

src/frontend/apiserver/backend/clib.go

@@ -213,6 +213,9 @@ func handleDataset(d *C.dataset_t) C.int {
 
 // handleConfigPush processes a cluster configuration push.
 func handleConfigPush(cfgdata *C.xce_configdata_t) C.int {
+	if gHandler == nil {
+		return 0
+	}
 	if cfgdata.data_which != C.ConfigData_data_clusterconfig {
 		return 0
 	}
@@ -222,14 +225,16 @@ func handleConfigPush(cfgdata *C.xce_configdata_t) C.int {
 		return 0
 	}
 
-	for _, q := range cClusterConfQueues(conf) {
-		qi := q
-		gHandler.OnQueueUpdated(&qi)
-	}
-	for _, r := range cClusterConfResources(conf) {
-		ri := r
-		gHandler.OnResourceUpdated(&ri)
+	cluster := cClusterConfToClusterInfo(conf)
+	if cluster == nil {
+		logger.Warn("handleConfigPush: cluster_config_t has empty clustername; skipping OnClusterUpdated")
 	}
+	dispatchConfigPush(gHandler,
+		cClusterConfQueues(conf),
+		cClusterConfResources(conf),
+		cClusterConfHostConfigs(conf),
+		cluster,
+	)
 	return 0
 }