Improve proximity scoring

While the current discrete scoring works, a more continuous decay function like `1.0 / (1.0 + distance)` for `_calculate_proximity_score` can provide a smoother and potentially more nuanced scoring for varying distances.

```suggestion
        return 1.0 / (1.0 + distance)
```

_Originally posted by @sourceant[bot] in https://github.com/sourceant/sourceant/pull/37#discussion_r2265384806_
            