fix(codegen): widen from the value expression, not the enclosing statement

should_zext_widen decides the widening direction from the narrow side's AST
node, but the return and assignment coercions passed the statement node.
A statement has no type of its own, so the literal rule and the annotated
variable rule were skipped and the value fell through to sext:
`def f() -> uint64: return 0xDEADBEEF` produced 0xFFFFFFFFDEADBEEF and the
caller compared against 0xDEADBEEF and failed.

Unwrap Return/Assign/AnnAssign/AugAssign/Expr to their value expression
before the direction rules run, so a non-negative literal wider than
INT32_MAX zero-extends (0xDEADBEEF -> 0xDEADBEEF) and a negative one keeps
its sign (-1 -> 0xFFFFFFFFFFFFFFFF).

App/lib/core/Gen/Expr/ExprGenOps.vp

@@ -144,6 +144,11 @@ novirtual class ExprGenOps(ExprGenBase.ExprGenBase):
           6. 其余 → sext。
         """
         if narrow_val is None or narrow_val.Ty is None: return 0
+        # Some call sites hand over the enclosing statement (return / assignment) instead of the
+        # value expression. The statement carries no type of its own, so rules 2/3/5 would miss a
+        # literal or an annotated variable and fall through to sext (`return 0xDEADBEEF` from a
+        # uint64 function became 0xFFFFFFFFDEADBEEF). Unwrap to the value expression first.
+        narrow_node = self._widen_value_node(narrow_node)
         narrow_bits: int = self.get_llvm_type_bits(narrow_val.Ty)
         if narrow_bits == 1: return 1
         # 2. 整数字面量：以字面量数值为准（`0xFFFFFFFF` 是 4294967295 > INT32_MAX，必须 zext）
@@ -170,6 +175,18 @@ novirtual class ExprGenOps(ExprGenBase.ExprGenBase):
         if narrow_node is not None:
             if self._node_type_unsigned(pool, narrow_node, trans) != 0: return 1
         return 0
+    # _widen_value_node - 取语句节点的值表达式（Return/Assign/AnnAssign/AugAssign/Expr），
+    # 其他节点原样返回。加宽方向的判定必须落在**有类型的值节点**上。
+    def _widen_value_node(self, node: ast.AST*) -> ast.AST*:
+        """Value expression of a statement node (Return/Assign/AnnAssign/AugAssign/Expr)."""
+        if node is None: return node
+        nk_wv: int = node.kind()
+        if nk_wv == ast.ASTKind.Return: return (ast.Return*(node)).value
+        if nk_wv == ast.ASTKind.Assign: return (ast.Assign*(node)).value
+        if nk_wv == ast.ASTKind.AnnAssign: return (ast.AnnAssign*(node)).value
+        if nk_wv == ast.ASTKind.AugAssign: return (ast.AugAssign*(node)).value
+        if nk_wv == ast.ASTKind.Expr: return (ast.Expr*(node)).value
+        return node
     # _cdefine_int_of - 取节点引用的 DefineT 常量值。支持两种引用形态：
     #   · 裸名       `N`
     #   · 模块属性   `mod.N`（跨模块经 .vpi 注册；常量名就是 at.attr）