Refactor mo_task_spawn() for the new scheduler

This commit refactors mo_task_spawn() to better align with the new O(1)
scheduler design. The task control block (tcb_t) now embeds its list node
directly, removing the need for an external allocation and simplifying the
ready-queue bookkeeping. The enqueue operation is also moved inside a
critical section to guarantee consistent state updates during task creation.

In addition, the previous “first task assignment” logic is removed. The
first created task is always the system idle task during startup, so this
special-case handling is no longer required.

These changes improve system consistency and allow task priority migration
to occur even before a newly created task runs for the first time.

Author: vicLin8712

kernel/task.c

@@ -747,8 +747,11 @@ int32_t mo_task_spawn(void *task_entry, uint16_t stack_size_req)
     tcb->id = kcb->next_tid++;
     kcb->task_count++; /* Cached count of active tasks for quick access */
 
-    if (!kcb->task_current)
-        kcb->task_current = node;
+    /* Binding ready queue node */
+    tcb->rq_node.data = tcb;
+
+    /* Push node to ready queue */
+    sched_enqueue_task(tcb);
 
     CRITICAL_LEAVE();
 
@@ -762,7 +765,6 @@ int32_t mo_task_spawn(void *task_entry, uint16_t stack_size_req)
 
     /* Add to cache and mark ready */
     cache_task(tcb->id, tcb);
-    sched_enqueue_task(tcb);
 
     return tcb->id;
 }