Fix EXF binary latitude orientation (north-to-south → south-to-north)

ERA5 NetCDF stores latitude decreasing (60→20 N, j=0=60N), but MITgcm
data.exf expects the binary to be south-to-north (lat0=20.0, lat_inc=0.25,
j=0=20N).  Without the flip, MITgcm interpolates model lat=26N by reading
binary row j≈24 which holds ERA5 lat=54N data (cold North Atlantic air),
creating a ~20°C air-sea temperature difference that drives hflux values
well above the EXF range-check threshold and causes the run to abort at it=0.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: fluidnumericsJoe

spectre_utils/mk_exf_conditions.py

@@ -26,6 +26,9 @@ def _open_var(working_directory, prefix, mitgcm_name, years, t1, t2):
     data_vars = list(ds.data_vars)
     if len(data_vars) == 1 and data_vars[0] != mitgcm_name:
         ds = ds.rename({data_vars[0]: mitgcm_name})
+    # ERA5 latitude is stored north-to-south (60→20 N). Flip to south-to-north
+    # so the binary layout matches data.exf: lat0=20.0, lat_inc=+0.25 (j=0=20N).
+    ds = ds.isel(latitude=slice(None, None, -1))
     return ds