Fix StackOverflowException in Precision.Increment/Decrement for count == int.MinValue

[sailro]

Fixes #1153.

Problem

Precision.Increment(double, int) and Precision.Decrement(double, int) throw an uncatchable StackOverflowException when count == int.MinValue. Each method delegates a negative count to its sibling with the negated count, and -int.MinValue overflows back to int.MinValue, so the two methods recurse into each other forever:

Precision.Increment(1.0, int.MinValue); // StackOverflowException
Precision.Decrement(1.0, int.MinValue); // StackOverflowException

Fix

Move the logic into private long-count overloads; the public int methods delegate to them. The negation is then performed in long, where -(long)int.MinValue == 2147483648 is representable, so a count of int.MinValue steps |int.MinValue| representable values in the opposite direction like any other negative count.

Behavior is unchanged for every other (value, count) — the public API is identical and the only previously-reachable difference is that int.MinValue crashed. I verified this with a differential check comparing the old and new implementations over a grid of boundary values (0, ±0.0, ±1, ±ε, MaxValue, MinValue, NaN, ±∞, …) × counts (0, ±1, ±2, ±5, ±1e6, ±int.MaxValue): bit-identical for every pair.

Test

Added PrecisionTest.IncrementDecrementWithIntMinValueCount, which asserts the int.MinValue case steps the bit pattern by 2147483648 in the correct direction (and, by completing at all, that it no longer overflows the stack). The full PrecisionTest class passes (162 tests).