In a class, an unique symbol attribute is sometime lost during incremental build

### 🔎 Search Terms

- unique property
- incremental build
- interface declaration merging

### 🕗 Version & Regression Information

- Tested with 6.0.2 and 6.0.3

### ⏯ Playground Link

[Playground Link](https://www.typescriptlang.org/play/?ts=6.0.0-dev.20260416#code/MYewdgzgLgBAygTQLICEDyAZAXDArmASwEdcBTGCATwFsAjEAGxgF54b6GAKASgG4AofsAYBDCBBgANGAG9BMBTAAOAJxBRSwDQBMYIqFBUsYAIhABrEwP6KYK0iO3gGlGAG1EqTAF1jZy9a2quqaOjAAZiAgPLI2tooA9Al4EOQEUHHxMFAAFgQQHsjoGN4CtgC+-JX8UJRK5AAqADwAcjCkAB4aYNoS0CoEYADmAHzGAEqaICraTQAGjtoAJDIt5XMANDAxzGMAbiAE2iPWgxoq4SLA5NKd3b0wzSY5IMAmYzLVoJCwHcafegkkgEHQAdIsXsAgA)

### 💻 Code

I do not have a minimal reproductible exemple as the issue seems to come from the incremental build.

I had a class with an unique symbol property, on which I was doing interface declaration merging:
```ts
const SYMBOL: unique symbol = Symbol();

class X {

protected attr = "ok";

readonly [SYMBOL] = "ok";

protected foo() {
// use it
this[SYMBOL];
}
}

type T = Record<`add${N}`, () => void>;

interface X extends T<"hoc"> {}

const x = {} as X;
x.addhoc
```

### 🙁 Actual behavior

The following error first appeared when I set some of the public attributes as protected (the error below is edited to match the provided exemple) :
```
ERROR in [...]
TS7053: Element implicitly has an 'any' type because expression of type 'unique symbol' can't be used to index type 'X'.
Property '[????.SYMBOL]' does not exist on type 'X'.
92 |
93 | protected foo() {
> 94 | this[SYMBOL];
| ^^^^^^^^^^^^^^^^
95 | }
```

I noticed that :
- The error disappear if I delete all of TS cache files.
- The error reappear if I rebuild without any changes to the files.
- The error reappear if I edit the files that defines `X`.
- The error does not reappear if I change other files.
- In watch mode, the error sometimes disappear and reappear when I change the files that defines `X`.
- The error doesn't appear in my VSC editor.

### 🙂 Expected behavior

No TS errors.

### Additional information about the issue

I also reproduced the error with `tsc --noEmit`, so it doesn't seem to come from my Webpack build system.

Bắt đầu với ví dụ Playground được liên kết và tái hiện nó bằng tsc --noEmit, sau đó so sánh các build sạch và build incremental trong khi thay đổi các tệp định nghĩa X; hoàn thành khi các lần rebuild lặp lại không còn tạo ra TS7053 cho việc truy cập này.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.