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/convert.go

@@ -10,7 +10,10 @@ import "C"
 
 import (
 	"apiserver/models"
+	logger "apiserver/utils"
 	"unsafe"
+
+	"go.uber.org/zap"
 )
 
 // goString safely converts a C string to a Go string, handling nil.
@@ -250,3 +253,52 @@ func freeJobSubmit(js *C.xce_job_submit_t, sd *C.job_submit_data_t, si *C.job_st
 		C.free(unsafe.Pointer(js))
 	}
 }
+
+// cResvalToDistribution converts a C xce_resval_t pointer to a Go ResourceDistributionInfo.
+// Returns nil if vp.val is NULL (nil guard prevents CGO panic).
+// vp.val is a *C.xce_val_t pointer field — pass directly, no &.
+func cResvalToDistribution(vp *C.xce_resval_t) *models.ResourceDistributionInfo {
+	rd := &models.ResourceDistributionInfo{
+		ResourceName: C.GoString(vp.name),
+		Location:     C.GoString(vp.host),
+		IsDynamic:    true, // resource values pushed from workers are always dynamic
+	}
+	// Guard: vp.val may be NULL if the controller sends a resval with no value set.
+	// Dereferencing NULL in xce_val_which() would segfault in C.
+	if vp.val == nil {
+		logger.Warn("resval with nil value, skipping",
+			zap.String("resource", rd.ResourceName),
+			zap.String("host", rd.Location))
+		return nil
+	}
+	switch C.xce_val_which(vp.val) {
+	case C.Value_u_ivalue32:
+		rd.ValueType = int(models.NUMERIC)
+		rd.NumericValue = float64(C.xce_val_i32(vp.val))
+	case C.Value_u_ivalue64:
+		rd.ValueType = int(models.NUMERIC)
+		rd.NumericValue = float64(C.xce_val_i64(vp.val))
+	case C.Value_u_uvalue32:
+		rd.ValueType = int(models.NUMERIC)
+		rd.NumericValue = float64(C.xce_val_u32(vp.val))
+	case C.Value_u_uvalue64:
+		rd.ValueType = int(models.NUMERIC)
+		rd.NumericValue = float64(C.xce_val_u64(vp.val))
+	case C.Value_u_fvalue:
+		rd.ValueType = int(models.NUMERIC)
+		rd.NumericValue = C.xce_val_fval(vp.val) // C.double → float64, no explicit cast needed
+	case C.Value_u_svalue:
+		rd.ValueType = int(models.STRING)
+		rd.StringValue = C.GoString(C.xce_val_str(vp.val))
+	case C.Value_u_bvalue:
+		rd.ValueType = int(models.BOOLEAN)
+		rd.BooleanValue = C.xce_val_bool(vp.val) != 0
+	default:
+		logger.Warn("unknown xce_val_t discriminant, skipping",
+			zap.Int("which", int(C.xce_val_which(vp.val))),
+			zap.String("resource", rd.ResourceName),
+			zap.String("host", rd.Location))
+		return nil
+	}
+	return rd
+}