Force doubles to ints in group()

PHP 8.4 deprecates the implicit conversion from doubles to ints when doing so would lose precision.
However, disallowing doubles as array keys in Functional would be a BC break.

src/Functional/Group.php

@@ -32,6 +32,11 @@ function group($collection, callable $callback)
 
         InvalidArgumentException::assertValidArrayKey($groupKey, __FUNCTION__);
 
+        // Avoid implicit precision-loss from doubles (which cannot be keys)
+        if (is_numeric($groupKey)) {
+            $groupKey = intval($groupKey);
+        }
+
         if (!isset($groups[$groupKey])) {
             $groups[$groupKey] = [];
         }