Make wrapping arithmetic unchecked

[SimonCropp]

Bug

Polyfill is compiled into the consuming project, so it inherits the consumer's CheckForOverflowUnderflow setting. Several polyfills rely on wrapping arithmetic without saying so, and throw OverflowException in a checked build:

• BitConverter.UInt32BitsToSingle, UInt64BitsToDouble, SingleToUInt32Bits, DoubleToUInt64Bits: signed/unsigned reinterpreting casts, e.g. SingleToUInt32Bits(-1f)
• BitOperations.IsPow2 (all six overloads, in both BitOperations.cs and BitOperationsIsPow2.cs): value & (value - 1) for 0u, int.MinValue, long.MinValue, ...
• RandomNumberGenerator.GetInt32(fromInclusive, toExclusive): (uint) casts of negative bounds, e.g. GetInt32(-10, -5)
• Random.NextInt64(minValue, maxValue): maxValue - minValue and the final cast/add for ranges wider than long.MaxValue, e.g. NextInt64(long.MinValue, long.MaxValue)

Fix

Wrap those expressions in unchecked(...). No behaviour change in unchecked builds.

Test

Added edge-value tests: the four BitConverter *_NegativeOne tests, BitOperations_IsPow2_NativeMinValue, GetInt32_Range_Negative, GetInt32_Range_Full, RandomNextInt64_NegativeRange and RandomNextInt64_FullRange (the existing IsPow2 tests already cover 0u and int.MinValue).

The test projects build unchecked, so these only fail before the fix in a checked build. Verified locally with -p:CheckForOverflowUnderflow=true on net48: 13 of the 21 selected tests failed with OverflowException before the fix and all pass after. In the normal build they pass on net48 and net8.0 (polyfill) and net10.0 (BCL). The BitOperationsIsPow2.cs copy is only active on netcoreapp3.0 to net6.0, whose runtimes are not installed locally, so that file is covered by CI only.