`mse` raises `UndefVarError: y_true` on any call

`src/SolePostHoc.jl`, line 14:

```julia
# Function mse (from Metrics.jl)
mse = (y_pred, t_true) -> (sum((y_true .- y_pred) .^ 2) / length(y_true))
```

The parameter is named `t_true`; the body uses `y_true`, which is not bound in that scope. Any call
raises `UndefVarError: y_true` rather than computing a mean squared error.

Looks like a single-character slip when the helper was written.

### What would close it

Renaming the parameter to `y_true` (or the body to `t_true`), plus a test that calls it - the defect
survives precisely because nothing exercises it.