feat: handle unequal cluster counts across slices (extremes)

[FBumann]

Problem

When using `ExtremeConfig(method="append")` with auto-sliced dims, different slices can produce different numbers of clusters (e.g., one scenario has a peak that triggers an extra extreme cluster, another doesn't). Currently this raises a ValueError.

Current behavior

# Raises ValueError: "Slices produced different cluster counts"

Possible improvements

1. Pad with NaN — the slice with fewer clusters gets NaN-filled extra clusters, allowing concat
2. Warn instead of error — let the user decide
3. Validate upfront — check if ExtremeConfig could cause mismatches and warn before aggregating

Context

See discussion in #13. The `method="replace"` workaround always works since it doesn't add clusters.