Prevent NaN centroid means in TDigestDouble (#702)

[PerumalsamyR]

Addresses #702

Problem

When merging a value into an existing centroid, the mean was updated incrementally:

centroidMeans_[i] += ((values[current] - centroidMeans_[i]) * weights[current]) / centroidWeights_[i];

The intermediate (value - mean) (or its product with the weight) can overflow to infinity even when both operands are finite — e.g. means near opposite ends of the double range. Once a centroid mean becomes infinite, a subsequent update produces Inf + (-Inf) = NaN, which is then stored, breaking the sort/binary-search invariants and getting serialized. The same overflow pattern existed in weightedAverage(), used for quantile interpolation.

This mirrors the fix already applied to the Rust implementation in apache/datasketches-rust#23.

Changes

• merge(): keep the incremental mean update in the common path (bit-identical results), but fall back to an overflow-safe weighted average m1*(w1/w) + m2*(w2/w) when the result is not finite. Each term of that form is bounded by the magnitude of its input, so the stored mean stays finite for finite inputs.
• weightedAverage(): normalize the weights before multiplying so quantile interpolation between centroids of large magnitude cannot overflow.
• update(): ignore ±Infinity in addition to NaN. With mixed infinities in the input, no update formula can keep centroid means well-defined, and rejecting them was the approach endorsed in the Rust PR discussion.
• heapify() / compat formats: validate deserialized state as requested in the issue — min/max, centroid means and buffered values must be finite, centroid weights must be positive — throwing SketchesArgumentException otherwise, since serialized input cannot be assumed to be well-formed.

Testing

• New test extremeValuesDoNotProduceNaN feeds 10k alternating ±Double.MAX_VALUE values: on current main it produces non-finite quantiles (and NaN centroid means); with this change all centroid means and quantiles stay finite.
• New tests for update() ignoring NaN/±Inf and for heapify() rejecting corrupted bytes (NaN/Inf mean, NaN min, zero weight, NaN single value).
• Full suite passes: mvn clean test — 2174 tests, 0 failures.