Refactor timer handler to support embedded list nodes

Previously, expired timers were placed into the expired_timers array and
their list nodes were returned to timer_node_pool. With embedded list
nodes, a timer only needs to be removed from the running timer list to
stop further handling.

This change updates timer handling to operate directly on embedded list
nodes, aligning the handler logic with the embedded node design.

kernel/timer.c

@@ -201,22 +201,21 @@ void _timer_tick_handler(void)
     if (unlikely(!timer_initialized || !kcb->timer_list ||
                  list_is_empty(kcb->timer_list)))
         return;
-
     uint32_t now = mo_ticks();
-    timer_t *expired_timers[TIMER_BATCH_SIZE]; /* Smaller batch size */
+    list_node_t *
+        expired_timers_running_nodes[TIMER_BATCH_SIZE]; /* Smaller batch size */
     int expired_count = 0;
 
     /* Collect expired timers in one pass, limited to batch size */
     while (!list_is_empty(kcb->timer_list) &&
            expired_count < TIMER_BATCH_SIZE) {
         list_node_t *node = kcb->timer_list->head->next;
-        timer_t *t = (timer_t *) node->data;
+        timer_t *t = timer_from_running_node(node);
 
         if (now >= t->deadline_ticks) {
-            expired_timers[expired_count++] = t;
-            kcb->timer_list->head->next = node->next;
-            kcb->timer_list->length--;
-            return_timer_node(node);
+            expired_timers_running_nodes[expired_count++] =
+                list_pop(kcb->timer_list);
+
         } else {
             /* First timer not expired, so none further down are */
             break;
@@ -225,7 +224,8 @@ void _timer_tick_handler(void)
 
     /* Process all expired timers */
     for (int i = 0; i < expired_count; i++) {
-        timer_t *t = expired_timers[i];
+        list_node_t *expired_running_node = expired_timers_running_nodes[i];
+        timer_t *t = timer_from_running_node(expired_running_node);
 
         /* Execute callback */
         if (likely(t->callback))
@@ -236,7 +236,8 @@ void _timer_tick_handler(void)
             /* Calculate next expected fire tick to prevent cumulative error */
             t->last_expected_fire_tick += MS_TO_TICKS(t->period_ms);
             t->deadline_ticks = t->last_expected_fire_tick;
-            timer_sorted_insert(t); /* Re-insert for next expiration */
+            /* Re-insert for next expiration */
+            timer_sorted_insert_running_list(t);
         } else {
             t->mode = TIMER_DISABLED; /* One-shot timers are done */
         }