fix: align generated nullable policy with consuming project

[ycherkes]

Make generated nullable handling project-level

Summary

AlephMapper-generated mapper files now follow the consuming project's NullableContextOptions rather than the source-local #nullable state of individual mapping methods or inlined helpers.

Each generated mapper file emits one file-level #nullable restore immediately after its auto-generated header. This restores the project's nullable configuration for Roslyn-generated source trees, which otherwise default to nullable-disabled behavior.

Changes

• Removed nullable directives from generated-member framing.
• Simplified NullablePolicy to map project-level Enable, Disable, Warnings, and Annotations modes.
• Kept nullable-aware type rendering driven by the project policy.
• Suppress nullable reference syntax, including class? constraints, when project annotations are disabled.
• Removed automatic null-forgiving operators from null-conditional Ignore and Rewrite output. Genuine nullable-flow diagnostics are no longer hidden by generated ! or null! syntax.
• Updated generated baselines and documented the behavior in the README.

Tests

• Added coverage for all four NullableContextOptions modes, independently modeling source annotations and expected generated annotations.
• Added a nullable-sensitive mixed-context helper test. It verifies helper directives are not propagated and that an expected generated CS8602 warning is not silently suppressed under project nullable Enable.
• Confirmed generated mapper output has a single file-level #nullable restore and no member-level nullable directives.

Nullable-flow behavior

• Enable: generated annotations are emitted and nullable-flow warnings may surface.
• Disable: generated annotations and nullable-flow warnings are disabled.
• Warnings: generated annotations are omitted while nullable-flow warnings may surface.
• Annotations: generated annotations are emitted while nullable-flow warnings are disabled.

The solution build now reports two legitimate CS8600 warnings from existing generated Experiment code. They are intentionally not suppressed: generated code follows the consuming project's nullable-warning policy.

Validation

• dotnet build AlephMapper.slnx --configuration Release
• dotnet run --project tests/AlephMapper.Tests/AlephMapper.Tests.csproj -c Release — 60 passed
• dotnet test AlephMapper.slnx --configuration Release — 97 passed
• dotnet pack source/AlephMapper.csproj --configuration Release --output artifacts/nuget
• Package smoke build using <Nullable>enable</Nullable>