Stop multiplying by zero from throwing when the zero is on the right [patch]

[matt-edmondson]

Fixes #108

What was wrong

LowestSignificantDigits, used by Multiply, Divide, Mod and CompareTo, read the left count after it had already overwritten it. When both operands were -1, 0 or 1, the result depended on operand order. A zero on the right gave a count of 0, and ToSignificantNumber(0) throws. So 1 * 0, 0 * 0 and default * default threw, while 0 * 1 returned 0.

Fix

• Capture both unlimited-precision flags before choosing a count.
• When both operands are exact, return 1. Any product, quotient or remainder of -1, 0 and 1 is exact at one significant digit.
• Otherwise, behaviour is unchanged: an exact operand defers to the other one's count, and two inexact operands take the lower count.

The same change makes CompareTo antisymmetric for these values. Before, 1.CompareTo(0) returned 0 while 0.CompareTo(1) returned -1.

Tests

The new SignificantNumberExactOperandTests covers:

• the repro lines from the issue
• Square(SignificantNumber.Zero) through generic math
• a * b == b * a == exact for every pair drawn from {-1, 0, 1}
• exact Divide/Mod results
• CompareTo sign for every pair

With the fix reverted, 7 of these fail. With it, all 120 tests pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_01QBrEwaLGeo2SvjVK3AnLrp

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[sonarqubecloud]

Quality Gate passed

Issues
0 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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