Silence xarray FutureWarning for merge compat parameter

xarray emits a FutureWarning when merging datasets without explicit `compat` parameter:

```
FutureWarning: In a future version of xarray the default value for compat will change from compat='no_conflicts' to compat='override'.
```

Update `Comparer.__add__` to explicitly pass `compat='no_conflicts'` to maintain current behavior.

File: `src/modelskill/comparison/_comparison.py` (around line 775)