feat: add TDigest, every quantile of a stream from one sketch

[alxkm]

Adds the t-digest: a small, mergeable sketch that answers any quantile of a stream, and its CDF, from one structure. It is the general counterpart of P2QuantileEstimator, which tracks a single quantile chosen in advance in constant memory.

The sketch is a list of centroids, each holding a mean and a weight, kept sorted by mean. What makes it more than a histogram is the rule that decides how large a centroid may grow: a centroid absorbs points only while it spans at most one unit of the scale function

k(q) = d / (2 pi) * asin(2q - 1)

where d is the compression. asin is steep near q = 0 and q = 1, so centroids in the tails are forced to stay tiny, often a single point, while those near the median are allowed to grow large. The rank error is of the order of 1 / d around the median and shrinks towards the ends, which is the trade latency percentiles want: p99.9 comes out nearly exact from a sketch of a few kilobytes, independent of the stream length.

Operation	Cost
add	O(log d) amortised
quantile, cdf	O(d)
merge	O(d log d)
memory	O(d)

A few decisions that are worth a reviewer's attention:

• Insertions are buffered and folded into the centroid list in batches, which is what keeps add cheap. Queries fold the pending buffer in before answering, so they mutate internal state; the class is documented as not thread-safe rather than pretending otherwise.
• The minimum and maximum are tracked separately and returned exactly for q = 0 and q = 1. Between them, quantile and cdf interpolate linearly across centroids, each treated as sitting at the centre of the weight it carries, with min and max closing the two ends.
• Digests merge the way partial sums do, so sketches built on separate shards combine into one. Merging a digest into itself gives the same result as merging an equal copy.
• Bad input is refused with an IllegalArgumentException: non-finite samples, invalid weights, quantiles outside [0, 1], a NaN passed to cdf, and a compression below 10, where the scale function stops leaving room for a useful number of centroids. Querying an empty digest is an IllegalStateException.

TDigestTest runs 52 cases across 25 methods. Accuracy is asserted against the true rank rather than eyeballed: on 100 000 uniform samples every tested quantile lands within 0.01 of the requested rank; on 200 000 Gaussian samples the rank error stays under a bound that narrows towards the tails, from 0.01 at the median to 0.0005 at q = 0.001 and q = 0.999. Three shards merged answer within 0.01 of the true rank, with total weight, minimum and maximum exact. The centroid count stays at or below the compression after 200 000 samples for compressions of 20, 100 and 500, with the centroids sorted and their weights summing to the total. Weighted, sorted, constant and heavily duplicated input, and wildly disparate weights, are covered as well.

Reference: T. Dunning, O. Ertl, Computing extremely accurate quantiles using t-digests.

Checklist

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new algorithms include a corresponding test class that validates their functionality.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.01980% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.17%. Comparing base (cb6f438) to head (fe31b7b).

Files with missing lines	Patch %	Lines
...main/java/com/thealgorithms/streaming/TDigest.java	98.01%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7642      +/-   ##
============================================
+ Coverage     82.04%   82.17%   +0.13%     
- Complexity     8286     8363      +77     
============================================
  Files           840      841       +1     
  Lines         25872    26074     +202     
  Branches       5053     5086      +33     
============================================
+ Hits          21227    21427     +200     
- Misses         3856     3858       +2     
  Partials        789      789              

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