fix(tdigest): interpolate quantiles toward the nearer centroid and normalize the left tail

[jaideeppyne]

Summary

• Correct the interior quantile interpolation weights so the estimate moves toward the nearer centroid as rank increases.
• Fix the mirrored tail formulas: subtract from max in the right quantile tail and normalize the left rank tail by total weight.
• Handle the weight-two right-tail boundary without evaluating an indeterminate 0 / 0 expression.
• Add focused exact regressions and monotonicity properties for directly updated and merged digests.
• Document the bug fix in the changelog.

Why

The interior interpolation paired each centroid mean with the distance from that same side of the bracket. Linear interpolation needs the opposite-side distance, so the previous result could move toward the lower centroid as rank increased and make quantiles non-monotonic.

The left and right tail branches contained two independent mirror errors. The right-tail formula also degenerates when the last centroid has weight two: at the only boundary that reaches the formula, both its numerator and denominator are zero. Returning the known maximum at that boundary keeps the handling local and avoids changing interpolation elsewhere.

Validation

• cargo x check
• cargo x test
• cargo x lint
• Exact interior, tail, and weight-two boundary regressions
• QuickCheck monotonicity properties for digests built through both update and merge

[tisonkun]

Thanks for your contribution! Merging ...