BUG: prevent masked values leaking into jackknife resamples - #20582

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Hi @giles, this pull request addresses #20579 where masked elements in a MaskedArray leaked into resampled jackknife slices and corrupted downstream estimates and standard errors.

I encountered this issue while running jackknife estimators on flagged astronomical time-series data. The PR includes the fix, comprehensive regression tests for MaskedArray inputs, and the towncrier changelog fragment. All test suites on CircleCI are green.

Could maintainers please reopen this PR for review?