Compute Pow and Exp beyond the range of a double [patch]

[matt-edmondson]

Fixes #106

What was wrong

Outside the exact integer path, Pow and Exp computed through double:

• A result above double's range threw ArgumentOutOfRangeException("Infinite values are not supported"). This hit 10.Pow(2000), Exp(710), and also 1e400.Pow(0.5), where the base itself converts to Infinity.
• A result below double's range silently became an exact 0. This hit 2.Pow(-1100), 1e-400.Pow(0.5) and Exp(-1000).

Fix

• The existing double computation still runs first. When its result is a normal double, nothing changes, so every in-range result is bit-for-bit what it was.
• When the result isn't normal (infinite, zero or subnormal), work in base-10 logarithms instead:
  • log10|x| comes from BigInteger.Log10(significand) + exponent, so a base outside double's range works too.
  • For Exp, log10(e^p) = p·log10(e).
  • The result is rebuilt as 10^fraction × 10^whole with the whole power kept as an int (CreateFromComponents), so the exponent never passes through a double.
• The result is rounded to the fewer of:
  • the usual significant-figure count
  • the digits the logarithm can actually determine, about 15 minus the digits in the integer part of the log. For Exp(±1000) that gives 12 digits, all of them correct.
• A power of ten beyond ±1e9 throws OverflowException naming the operation, instead of returning infinity or zero. The Pow/Exp docs list the new exception.

Tests

The new SignificantNumberRangeTests covers every call in the issue's table, plus:

• negative bases with odd powers in both directions
• 1e400.Pow(-0.5)
• checks of Exp(1000) and Exp(-1000) to 12 digits against high-precision reference values
• OverflowException for extreme exponents in both directions

One correction to the issue: 1.5.Pow(2000) is 1.52e352, not ≈1.4e352, and the test expects 1.5E352.

With the fix reverted, all 12 new tests fail. With it, the suite passes, and the full solution builds clean for net7.0 through net10.0. This branch merges cleanly with #110 and #111.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QBrEwaLGeo2SvjVK3AnLrp

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Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
95.5% Coverage on New Code
0.0% Duplication on New Code

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