Add opt-in nullability-aware null generation, fix null thresholds and date ranges

[holonsoftFounder]

Feature: nullability-aware null generation (opt-in)

x.RespectNullableAnnotations(threshold) on the configuration builder. When enabled, every property or field declared as nullable (string?, int?, DateTime?, ...) gets its data source wrapped in a NullableMemberDataSource that returns null with the given probability (default AutoPocoGlobalSettings.NullCreationThreshold, 15 percent).

• Covers convention sources, explicit Setup(...) sources and generation time Source(...) overrides.
• Impose, non-nullable members, method parameters and members of types compiled without nullable annotations are never touched.
• Each member gets its own deterministic seed (FNV-1a of the member key xor the standard seed), so the nullable members of one object do not all become null at once and two sessions of one factory produce the same pattern.
• Plumbing: GenerationSessionFactory and GenerationConfiguration take optional NullableAnnotationSettings, exposed as IGenerationConfiguration.NullableAnnotations; ObjectBuilder wraps per member.
• IDataSource<T> is covariant now, so .Setup(c => c.City).Use<CitySource>() compiles without a nullability warning for a string? member.

Bug fixes

• Every Nullable* source ignored an explicit null creation threshold. The fixed array and dictionary sources even used it as the random seed. DataSourceBase got a protected constructor that installs the evaluator with the threshold; all nullable source bases use it. Null positions in the stable sequences of those sources changed, the data order did not.
• DateTimeSource, DateOnlySource, TimeOnlySource, DateOfBirthSource: exclusive upper bounds (no December, no 31st, no 23:00, no :59, max year unreachable) and ranges within one year could yield values outside the range. All four pick uniformly from the inclusive range now, validate min <= max, TimeOnly wraps around midnight, DateTime keeps the kind of the minimum date. Sequences for a given seed changed.
• RandomUtfTextSource looped forever when it hit a Unicode block without allowed characters (found by the new theory that draws 30 values per source).

Verification

• dotnet build -c Release: zero warnings on net8/9/10.
• dotnet test: 683 runs per framework, all green on net8.0, net9.0, net10.0 (Windows) and net10.0 on Linux (mcr.microsoft.com/dotnet/sdk:10.0 container).
• README "New in 6.0.0" and PackageReleaseNotes updated.

🤖 Generated with Claude Code