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Preserve streaming covariance precision near large offsets - #5
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Paired Welford updates on observations
(1e16, 1e16)and(1e16 + 2, 1e16 + 2)returned sample covariance 4 instead of the exact value 2: the large-coordinate midpoint cannot be represented as a float.Shift each observation by the first pair before updating the running means and cross-moment. This keeps O(1) streaming state and preserves small deviations near large common offsets. Add exact translation-based regressions for positive and negative covariance and both offset signs; retain the existing standard-library oracle for moderate data.
Validation: 303 tests pass; all doctests, Ruff lint, and formatting pass locally.