Change `return` to `$result` for method return values in refinements

@rajshivu 已经在做这个了。

开始于 2026年2月9日。

• #145 来自 @rajshivu —— 未关闭

`return` is a statement, and it's strange to have it as a variable to represent the return value.
Therefore, we could change it for `result`. But since result can be an actual variable that represents something other than the return value we, we can try using `$result`.

This would mean a small change in the grammar, probably, and a change in replacing the return value.
Ps: the _ should still work in any case

Example Before:
```java
@Refinement("return > 0")
int test2 (){
return 10;
}
```
Example After:
```java
@Refinement("$result > 0")
int test2 (){
return 10;
}
```

首先定位 refinement grammar，以及替换或检查返回值的代码；issue 提到了两者，但没有指出文件或测试。将现有对 `return` 和 `_` 的处理方式与提议的 `$result` 语法进行比较，并在开始前考虑已打开的 pull request。完成的标准是新语法能够正常工作，且不会破坏现有行为。

由索引模型根据 Issue 内容生成。