Refactor list node to support embedded usage

Remove the data member from list_node to support an embedded-node design.
List nodes are now embedded directly into container structures, eliminating
the need to manage list node lifecycles during pushback or remove operations.
The lifecycle is instead owned by the container itself.

This change updates existing list helpers to align with the embedded design
and reduces unnecessary memory operations during list manipulation.

Author: vicLin8712

include/lib/list.h

@@ -15,7 +15,6 @@
 /* List node */
 typedef struct list_node {
     struct list_node *next;
-    void *data;
 } list_node_t;
 
 /* Public list descriptor */
@@ -45,10 +44,8 @@ static inline list_t *list_create(void)
     }
 
     head->next = tail;
-    head->data = NULL;
 
     tail->next = NULL;
-    tail->data = NULL;
 
     list->head = head;
     list->tail = tail;
@@ -78,16 +75,11 @@ static inline list_node_t *list_cnext(const list_t *list,
 
 /* Push and pop */
 
-static inline list_node_t *list_pushback(list_t *list, void *data)
+static inline list_node_t *list_pushback(list_t *list, list_node_t *node)
 {
-    if (unlikely(!list))
-        return NULL;
-
-    list_node_t *node = malloc(sizeof(*node));
-    if (unlikely(!node))
+    if (unlikely(!list || !node || node->next))
         return NULL;
 
-    node->data = data;
     node->next = list->tail;
 
     /* Insert before tail sentinel */
@@ -100,22 +92,21 @@ static inline list_node_t *list_pushback(list_t *list, void *data)
     return node;
 }
 
-static inline void *list_pop(list_t *list)
+static inline list_node_t *list_pop(list_t *list)
 {
     if (unlikely(list_is_empty(list)))
         return NULL;
 
     list_node_t *first = list->head->next;
     list->head->next = first->next;
+    first->next = NULL;
 
-    void *data = first->data;
-    free(first);
     list->length--;
-    return data;
+    return first;
 }
 
-/* Remove a specific node; returns its data */
-static inline void *list_remove(list_t *list, list_node_t *target)
+/* Remove a specific node form the list */
+static inline list_node_t *list_remove(list_t *list, list_node_t *target)
 {
     if (unlikely(!list || !target || list_is_empty(list)))
         return NULL;
@@ -128,10 +119,9 @@ static inline void *list_remove(list_t *list, list_node_t *target)
         return NULL; /* node not found */
 
     prev->next = target->next;
-    void *data = target->data;
-    free(target);
+    target->next = NULL;
     list->length--;
-    return data;
+    return target;
 }
 
 /* Iteration */