Celestial meridian labels incorrect for eck4 projection

I am testing the following eck4 meridian labeling example:

```
def label_bug(celestial=True, lon_0=65):
m = Basemap(projection='eck4', lon_0=lon_0, resolution=None, celestial=celestial)
m.drawmeridians([lon_0, lon_0 + 60], labels=[0,0,1,0], labelstyle='+/-')
m.drawmapboundary()
```

With normal coordinates, the labels appear where I expect them:

```
label_bug(celestial=False)
```

When celestial is True, the meridians are correctly flipped, but the labels no longer match their positions:

```
label_bug(celestial=True)
```

If anyone knows a workaround until this is fixed, please let me know. Thanks!

Start with the supplied label_bug reproduction and Basemap.drawmeridians, comparing celestial=False and celestial=True for the eck4 projection. Trace how meridian positions and labels are transformed, then verify that labels match their displayed meridians in both cases using the example.

Written by the indexing model from the issue text.