fix: use value equality for sortByField tie detection

ClosureExpressionVisitor::sortByField() decided ties with a strict
"===" identity check before falling through to the next sort field.
For object-typed fields (e.g. two DateTimeImmutable instances that
represent the same point in time but are not the same instance),
this check never passes, so secondary/tertiary Criteria::orderBy()
fields were silently ignored and the resulting comparator was not
antisymmetric (swapping the input order of two such elements changed
the sort result).

Replace the "===" tie check with the same "<=>" comparison already
used to determine ordering, so a tie is now detected whenever the
values are equal under that comparison, matching value semantics for
both scalars and comparable objects.

Fixes #437

src/Expr/ClosureExpressionVisitor.php

@@ -170,11 +170,13 @@ class ClosureExpressionVisitor extends ExpressionVisitor
             $aValue = ClosureExpressionVisitor::getObjectFieldValue($a, $name, $accessRawFieldValues);
             $bValue = ClosureExpressionVisitor::getObjectFieldValue($b, $name, $accessRawFieldValues);
 
-            if ($aValue === $bValue) {
+            $comparison = $aValue <=> $bValue;
+
+            if ($comparison === 0) {
                 return $next($a, $b);
             }
 
-            return ($aValue > $bValue ? 1 : -1) * $orientation;
+            return $comparison * $orientation;
         };
     }