Keep NextDouble(min, max) below max and finite for wide bounds [patch]

[matt-edmondson]

Fixes #61

What was wrong

• IRandomProvider.NextDouble(min, max) computed min + u * (max - min). When the unit draw was close to 1, rounding could land exactly on maxExclusive. For example, NextDouble(1.0, BitIncrement(1.0)) returned max about half the time.
• Finite bounds of opposite sign overflowed the width, so NextDouble(double.MinValue, double.MaxValue) returned Infinity, or NaN when the unit draw was 0.
• UniformDistributionProvider had the same width overflow, which broke Mean, Median, Cdf, Quantile, Pdf and Sample.

Change

• In NextDouble(min, max), when the width is infinite it interpolates as min*(1-u) + max*u instead. If the result reaches max (and max > min), it returns the next double below max.
• Added RandomHelpers.NextDown, because Math.BitDecrement is not available on netstandard2.1.
• In UniformDistributionProvider:
  • Mean is now Min/2 + Max/2.
  • Pdf now uses a half width, which stays finite.
  • Cdf and Quantile switch to half-width and interpolation forms when the full width overflows.
  • Median follows from Quantile, and Sample follows from NextDouble.
  • Results for ordinary intervals are unchanged.

Tests

New RandomRangeBoundaryTests uses a constant-word stub provider:

• At the top draw, NextDouble stays below max for (1, 1+ulp), (1, 2), (0.3, 0.7), (5, 5.5) and (0.9, 1).
• NextDouble(MinValue, MaxValue) stays finite and below max for the draws 0, 2^63 and ulong.MaxValue.
• Uniform(-1e308, 1e308) gives Mean == 0, Median == 0, Cdf(0) == 0.5, Quantile(0.5) == 0, a positive Pdf, and a finite Sample.

With the fix reverted all 9 cases fail; with it they pass. The full solution builds for every target framework, including netstandard2.1. The full suite passes: 935 tests, 0 failed.

This PR stands alone on main. It also test-merges cleanly with #92 (the related #60 fix), which touches a neighbouring line in RandomHelpers.cs.

🤖 Generated with Claude Code

https://claude.ai/code/session_01ABXREnMPNwAYQMdr4STcfE

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Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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