Add intrusive node helpers for scheduler ready queues

This commit introduces two helper functions for intrusive list
usage, where each task embeds its own list node instead of relying
on per-operation malloc/free.

The new APIs allow the scheduler to manipulate ready-queue nodes
directly:

 - list_pushback_node(): append an existing node to the end of the
   list (before the tail sentinel) without allocating memory.

 - list_remove_node(): remove a node from the list without freeing
   it, allowing the caller to control the node's lifetime.

These helpers will be used by the upcoming O(1) scheduler
enqueue/dequeue paths, which require embedded list nodes stored in
tcb_t.

Author: vicLin8712

include/lib/list.h

@@ -100,6 +100,24 @@ static inline list_node_t *list_pushback(list_t *list, void *data)
     return node;
 }
 
+/* Pushback list node into list */
+static inline void list_pushback_node(list_t *list, list_node_t *target)
+{
+    if (unlikely(!list || !target || target->next))
+        return;
+
+    target->next = list->tail;
+
+    /* Insert before tail sentinel */
+    list_node_t *prev = list->head;
+    while (prev->next != list->tail)
+        prev = prev->next;
+
+    prev->next = target;
+    list->length++;
+    return;
+}
+
 static inline void *list_pop(list_t *list)
 {
     if (unlikely(list_is_empty(list)))
@@ -134,6 +152,25 @@ static inline void *list_remove(list_t *list, list_node_t *target)
     return data;
 }
 
+/* Remove a node from list without freeing */
+static inline void list_remove_node(list_t *list, list_node_t *target)
+{
+    if (unlikely(!list || !target || list_is_empty(list)))
+        return;
+
+    list_node_t *prev = list->head;
+    while (prev->next != list->tail && prev->next != target)
+        prev = prev->next;
+
+    if (unlikely(prev->next != target))
+        return; /* node not found */
+
+    prev->next = target->next;
+    target->next = NULL;
+    list->length--;
+    return;
+}
+
 /* Iteration */
 
 /* Callback should return non-NULL to stop early, NULL to continue */