RoundRNE is not rounding to even, both logic and value side

### Describe the bug

While the FP components match our conversion from double with rounding, the rounding algorithm is not truly RNE. RNE needs to consider the mantissa width; it looks like it is working correctly for odd lengths but not even.

### To Reproduce

The bug won't reproduce on 32-bit because the RoundRNE is correct for odd mantissa lengths. So it needs to be validated by inspection and modifying both the value side and the logic side together.

### Expected behavior

_No response_

### Actual behavior

_No response_

### Additional: Dart SDK info

_No response_

### Additional: pubspec.yaml

```yaml

```

### Additional: Context

_No response_