fix: return 0 for constant input in incr/skewness and incr/kurtosis

[apoorva-01]

Resolves #287.

Description

• Fixes @stdlib/stats/incr/skewness and @stdlib/stats/incr/kurtosis returning NaN instead of 0 when every provided value is equal.

The accumulators divide by a power of the second central moment M2 when computing the population moment. For constant input, M2 is exactly 0, so the computation is a 0/0 division (sqrt(N)*M3 / pow(M2, 1.5) for skewness; N*M4 / (M2*M2) for kurtosis), yielding NaN. The expected value for a constant data set is 0 (same as the issue's pandas reference, pandas-dev/pandas#18044). The fix guards against a zero M2 in both accumulators, in both the running-update and no-argument query paths.

Scope: these two are the only stats/incr packages performing this M2 division (no incr/m* variants, no incr/nankurtosis). incr/nanskewness wraps incr/skewness and inherits the fix; incr/summary consumes both and its suite still passes.

Related Issues

• Incremental skewness and other higher moments when provided equal values do not yield expected results #287

Questions

• I return 0.0 for the all-equal case in both skewness and excess kurtosis. Skewness 0 matches the issue; excess kurtosis 0 is the analogous convention for a constant series — happy to adjust if you'd prefer a different sentinel for kurtosis.
• Scoped to the two accumulators in one PR; can split per package if you'd rather.

Other

Tested: reproduced the MWE (100 × 10.0) → NaN before / 0 after for skewness, kurtosis, and nanskewness. Added a regression test to each package (asserting both return paths give 0), proven to fail when the main.js guard is reverted. Suites: skewness 34/34, kurtosis 36/36, nanskewness 16/16, summary 6/6.

Checklist

• Read, understood, and followed the contributing guidelines

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