fix(narrow): ClassGenEnum pos_max 加宽为 sizet

pos_max 承接 variant_field_sizes 的 sizet 元素，仅与 sizet 比较并流入 sizet()；原 int 触发 64->32 隐式窄化

App/lib/core/Gen/Class/ClassGenEnum.vp

@@ -738,7 +738,7 @@ novirtual class ClassGenEnum(ClassGenBase.ClassGenBase):
 
         # Positions 1+: take the max field size across all variants at each position
         for pos in range(1, total_positions):
-            pos_max: int = 0
+            pos_max: sizet = 0
             for vi in range(variant_count):
                 if vi >= 64:
                     break  # variant_field_sizes 64-class slot cap; guards against out-of-bounds reads