Migrate Digit, Divide, and ExceptNulls off Option

[KaliCZ]

Summary

• Digit now follows the TryCreate / Create pattern (no Option), lives under src/StrongTypes/Digits/, and implements IEquatable + IComparable against itself, byte, and int.
• Divide returns decimal?; SafeDivide is a thin Divide(b) ?? otherwise wrapper. Both live in the new src/StrongTypes/Numbers/NumberExtensions.cs.
• ExceptNulls is reintroduced as an NRT-aware extension in src/StrongTypes/Collections/IEnumerableExtensions.cs (where T : struct on IEnumerable<T?> and where T : class on IEnumerable<T?>). FilterDigits now uses it in place of Flatten(Option<T>).
• Legacy Digit_Old, DigitExtensions_Old, NumberExtensions_Old and their _Old tests are retired; the duplicate ExceptNulls is removed from the legacy IEnumerableExtensions_Generic_Old partial.

On record struct

I kept Digit as a plain readonly struct. A record struct would auto-generate Equals(Digit) / GetHashCode / == / !=, but we still need to hand-write IEquatable<byte>, IEquatable<int>, all three IComparable<T> overloads, the ordering operators, and a custom ToString() (the synthesized record ToString prints Digit { Value = 5 }). Once you add all that, the savings are small, and the remaining surface reads more uniformly against the other strong types (NonEmptyString, the numeric wrappers) when it's a regular struct with explicit equality.

Test plan

• dotnet build clean (0 warnings, 0 errors)
• dotnet test src/StrongTypes.Tests/StrongTypes.Tests.csproj — 470 passed
• Integration-test suite not run locally (requires Docker/Testcontainers)