Tweak distribution of diskann-wide float tests.

[Mark Hildebrand (hildebrandmw)]

The Finite distribution for generating floating point numbers in the diskann-wide random tests is slightly biased: the same value is used to select the kind of floating-point number and the value of that number. Practically speaking, the bias is relatively small - but it does mean that some valid floating-point bit patterns cannot be created.

This PR fixes that by splitting the kind generation from the value generation. As a side-effect, it also removes the tiny bias introduced by the modulo. This does mean that normal numbers have a slightly higher fraction of the generated values, but not significantly.

[Copilot]

Pull request overview

This PR updates the diskann-wide floating-point Finite test distribution to remove an unintended bias caused by reusing the same random bits for both “kind selection” (normal/subnormal/zero) and the generated value’s bit-pattern.

Changes:

• Split “kind selection” entropy from the generated float’s bit-pattern by sampling a wider integer and partitioning it into upper/lower bits.
• Replace the old non-power-of-two modulo weighting (100) with a power-of-two bucket count (128) to eliminate modulo-induced bias.
• Update the distribution tests’ expected weights to match the adjusted bucket thresholds.

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[Codecov Comments Bot (codecov-commenter)]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.27%. Comparing base (bea135b) to head (f2ca48a).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1305      +/-   ##
==========================================
- Coverage   91.46%   91.27%   -0.20%     
==========================================
  Files         516      517       +1     
  Lines       98200    98517     +317     
==========================================
+ Hits        89820    89919      +99     
- Misses       8380     8598     +218     

Flag	Coverage Δ
miri	91.27% <100.00%> (-0.20%)	⬇️
unittests	90.94% <100.00%> (-0.20%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
diskann-wide/src/test_utils/distribution.rs	100.00% <100.00%> (ø)

... and 14 files with indirect coverage changes

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.