stos: mm: Add virtual page allocation

Signed-off-by: Chloe M. <chloe@mensia.org>

paw/stos/mm/vad.c

@@ -0,0 +1,54 @@
+/*
+ * Copyright (c) 2026, Chloe M.
+ * Provided under the BSD-3 clause.
+ *
+ * Description: VAD management
+ * Author:      Chloe M.
+ */
+
+#include <mm/vad.h>
+
+VOID
+MmVadListAppend(MM_VAD_LIST *List, MM_VAD *Vad)
+{
+    MM_VAD *Last;
+
+    if (List == NULL || Vad == NULL) {
+        return;
+    }
+
+    Vad->Next = NULL;
+    Vad->Prev = NULL;
+
+    if (List->Last == NULL || List->First == NULL) {
+        List->Last = Vad;
+        List->First = Vad;
+    } else {
+        Last = List->Last;
+        Vad->Prev = Last;
+        Last->Next = Vad;
+        List->Last = Vad;
+    }
+}
+
+MM_VAD *
+MmVadListPop(MM_VAD_LIST *List)
+{
+    MM_VAD *Last;
+
+    if (List == NULL) {
+        return NULL;
+    }
+
+    if (List->Last == NULL) {
+        return NULL;
+    }
+
+    Last = List->Last;
+    if (Last->Prev != NULL) {
+        Last->Prev->Next = NULL;
+    }
+
+    List->Last = Last->Prev;
+    return Last;
+}