feat: P1-1 handle resource value push via OPCode_notify

Add OPCode_notify handler in go_push_handler top-level switch
(not inside handlePush) to receive resource value updates from
the controller via xce_notify_data_t/schedUpdateResvals.

- Add 8 CGO union accessors for xce_val_t (which, i32, i64,
  u32, u64, fval, str, bool) since CGO cannot access union fields
- Add handleNotifyData() dispatching NotifyData_data_schedUpdateResvals
  events, iterating xce_resval_list_t and calling cResvalToDistribution
- Add cResvalToDistribution in convert.go with 7 separate switch
  cases for Value_u_* union discriminants (no combining i32/i64)
- Add rawResvalData + rawResvalToDistribution pure-Go version
  in convert_pure.go for testability without CGO
- Add 10 TDD tests covering all 7 value types, nil val guard,
  unknown discriminant, and boolean false
- All 42 backend tests passing

src/frontend/apiserver/backend/clib.go

@@ -48,6 +48,16 @@ static void jobctrl_set_siginfo(xce_jobctrl_t *ctrl, xce_signal_t *sig) {
 static void jobctrl_set_ckptoption(xce_jobctrl_t *ctrl, xce_ckpt_options_t *ckpt) {
     ctrl->u.ckptoption = ckpt;
 }
+
+// xce_val_t union accessors — CGO cannot access C union fields by name.
+static int      xce_val_which(xce_val_t *v)  { return v->which; }
+static int32_t  xce_val_i32(xce_val_t *v)    { return v->u.ivalue32; }
+static int64_t  xce_val_i64(xce_val_t *v)    { return v->u.ivalue64; }
+static uint32_t xce_val_u32(xce_val_t *v)    { return v->u.uvalue32; }
+static uint64_t xce_val_u64(xce_val_t *v)    { return v->u.uvalue64; }
+static double   xce_val_fval(xce_val_t *v)   { return v->u.fvalue; }
+static char*    xce_val_str(xce_val_t *v)    { return v->u.svalue; }
+static int      xce_val_bool(xce_val_t *v)   { return v->u.bvalue; }
 */
 import "C"
 
@@ -160,6 +170,15 @@ func go_push_handler(req *C.xce_request_t, _ unsafe.Pointer) C.int {
 	switch req.opcode {
 	case C.OPCode_push:
 		return handlePush(req)
+	case C.OPCode_notify:
+		if req.body_which != C.Request_body_notifymsg {
+			return 0
+		}
+		nd := (*C.xce_notify_data_t)(unsafe.Pointer(C.xce_request_body(req)))
+		if nd == nil {
+			return 0
+		}
+		return handleNotifyData(nd)
 	}
 	return 0
 }
@@ -238,6 +257,36 @@ func handleConfigPush(cfgdata *C.xce_configdata_t) C.int {
 	return 0
 }
 
+// handleNotifyData processes an OPCode_notify request.
+// xce_notify_data_t uses 'event' (not 'data_which') as its union discriminant.
+func handleNotifyData(nd *C.xce_notify_data_t) C.int {
+	if nd == nil {
+		return 0
+	}
+	switch nd.event {
+	case C.NotifyData_data_schedUpdateResvals:
+		rvl := (*C.xce_resval_list_t)(C.xce_notify_data_ptr(nd))
+		if rvl == nil {
+			return 0
+		}
+		n := int(rvl.count)
+		if n == 0 || rvl.vals == nil {
+			return 0
+		}
+		vals := unsafe.Slice(rvl.vals, n)
+		for _, vp := range vals {
+			if vp == nil {
+				continue
+			}
+			rd := cResvalToDistribution(vp)
+			if rd != nil {
+				gHandler.OnResourceLocationUpdated(rd)
+			}
+		}
+	}
+	return 0
+}
+
 // SubmitJob sends a job submission to the backend via xcecli_jobsubmit.
 // arraySize=1 for normal jobs, >1 for array jobs.
 // The store is updated only when the backend pushes the resulting OnJobCreated