CC: Objects not properly tracked

## Compiler version

`3.8.1-RC1-bin-20251204-241f261-NIGHTLY`

## Minimized code

```scala
import language.experimental.captureChecking

def foo(c: AnyRef^, d: AnyRef^, e: AnyRef^): Unit =
  object O:
    val x = c
    def foo = println(d)

  val w: O.type = O // O should not be pure, but O^{c,d} 
```

## Output

It's happily accepted by the capture checker.

## Expectation

It shouldn't!