feat: fill scheduler controller message gaps

Wire up 6 previously-stubbed message handlers in the scheduler:
resource dataset, resval dataset, job update notify, queue update
notify, resval update notify, and job rusage notify. Remove the
unreachable pushmsg branch (vtask_entry rewrites all pushmsg to
datamsg before the handler fires).

Changes by component:
- helper.h/helper.c: add xce_notify_event_name() for symbolic log names
- scheduler.h: add 5 new xce_sched_op_t values and matching union fields
- plugin.h/plugin.c: add plugin_job_rusage_hook_t and plugin_job_rusage_update()
- schedobj.c: add sched_update_existing_queue() with NULL/-ENOENT guards
- scheduler.c: wire 5 new dequeue cases; add sched_update_existing_job()
  and static sched_op_name(); emit WARN log on handler failure
- handler.c: implement sched_handle_resources/resvals (dataset) and
  sched_handle_sched_update_{job,queue,resvals}/job_rusage (notify);
  remove dead sched_handle_jobpush/workerpush and pushmsg branch;
  update TRACE logs to use symbolic name helpers
- test.c: 5 new unit tests covering notify event names, enum values,
  and queue update NULL guard (15/15 pass)

src/lib/helper.c

@@ -1,4 +1,5 @@
 #include <errno.h>
+#include <stdio.h>
 #include <stdlib.h>
 
 #include "controller.capnp.h"
@@ -330,3 +331,55 @@ char* xce_dataset_table(int table)
 
     return "unknown";
 }
+
+char* xce_notify_event_name(int event)
+{
+    static char buf[32];
+
+    switch (event) {
+    case NotifyData_data_jobPidInfo:
+        return "jobPidInfo";
+    case NotifyData_data_jobFailInfo:
+        return "jobFailInfo";
+    case NotifyData_data_jobDispatchFail:
+        return "jobDispatchFail";
+    case NotifyData_data_jobFinishInfo:
+        return "jobFinishInfo";
+    case NotifyData_data_schedStartJob:
+        return "schedStartJob";
+    case NotifyData_data_schedExitJob:
+        return "schedExitJob";
+    case NotifyData_data_schedFailJob:
+        return "schedFailJob";
+    case NotifyData_data_schedUpdateJob:
+        return "schedUpdateJob";
+    case NotifyData_data_schedUpdateResvals:
+        return "schedUpdateResvals";
+    case NotifyData_data_stoppedJob:
+        return "stoppedJob";
+    case NotifyData_data_resumedJob:
+        return "resumedJob";
+    case NotifyData_data_ckptinfo:
+        return "ckptinfo";
+    case NotifyData_data_schedUpdateQueue:
+        return "schedUpdateQueue";
+    case NotifyData_data_taskPidInfo:
+        return "taskPidInfo";
+    case NotifyData_data_taskFailInfo:
+        return "taskFailInfo";
+    case NotifyData_data_taskFinish:
+        return "taskFinish";
+    case NotifyData_data_postDoneJob:
+        return "postDoneJob";
+    case NotifyData_data_schedJobRusages:
+        return "schedJobRusages";
+    case NotifyData_data_updateJob:
+        return "updateJob";
+    default:
+        break;
+    }
+
+    snprintf(buf, sizeof(buf), "UNKNOWN(%d)", event);
+    return buf;
+}
+