Fix swapped interpolation weights and right tail sign in tdigest Quantile

[jaideeppyne]

Two spots in tdigest.Double.Quantile differ from the reference implementation (MergingDigest). The first affects ordinary use, the second needs a decoded sketch.

The interpolation weights are passed in the wrong order

Reference:

double z1 = index - weightSoFar - leftUnit;
double z2 = weightSoFar + dw - index - rightUnit;
return weightedAverage(mean[i], z2, mean[i + 1], z1);

We compute the same two quantities as w1/w2 and then call weightedAverage(d.centroids[i].mean, w1, d.centroids[i+1].mean, w2). The weight on mean[i] should be the distance to the far end of the bracket, not the near one. As the rank rises inside a bracket the estimate moves toward the lower centroid instead of the upper one.

That makes Quantile non-monotonic from a plain Update sequence. 2000 of 2000 random digests return some Quantile(r2) < Quantile(r1) with r2 > r1, k in {10, 20, 50, 100, 200}, n in [100, 50000], gaussian / uniform / lognormal. It also shows on the checked-in fixtures: tdigest_ref_k100_n10000_double.sk and every serialization_test_data/cpp_generated_files/tdigest_double_* file give thousands of descents over 5000 evenly spaced ranks, and 0 after.

It costs accuracy too. Rank error against the exact quantiles of the input, 29970 queries at k=100, n=20000 gaussian:

          mean       max
before    0.011803   0.042200
after     0.000385   0.002100

The right tail branch adds where the reference subtracts

Reference is max - (...), we have d.max + (...), so the branch returns quantiles above MaxValue(). On min 0, centroids (10,100), (20,100), (30,100), max 40, rank 0.90 returns 45.92 where the reference gives 34.08.

This one is not reachable from Update or Merge. merge never lets the first or last centroid absorb a neighbour, so both tails stay singletons and lastWeight > 1 is false. I looked for a counterexample over 3000 digests built from random updates, merges and encode/decode round trips and never saw a first or last centroid of weight above 1. It is reachable from DecodeDouble, which accepts one, so the test builds it that way.

Rank's left tail already has the / centroidsWeight normalizer, so there is nothing to change there. That is the one difference from apache/datasketches-java#755, which fixes the same two plus a missing normalizer in getRank.

Two new tests, each verified failing before its own fix with the tests kept in place. go test ./... and go test -race ./tdigest/ are green before and after, so nothing existing covered this.

I used Claude Code on this. The oracles were tdunning's MergingDigest and monotonicity, which needs no second implementation. Differential testing against the C++ core would not have found it, since C++ has the same two.