Generated files stop compiling in C# 7.3 projects (CS8370 at line 3) because WriteFileHeader always emits #nullable enable #59

What's wrong

Since #55 / PR #57, WriteFileHeader writes #nullable enable into every generated file, whatever the consuming project's language version is:

#nullable needs C# 8. A project on C# 7.3 now fails to build with:

error CS8370: Feature 'nullable reference types' is not available in C# 7.3. Please use language version 8.0 or greater.

The error is reported at line 3 of every generated file. C# 7.3 is the default for netstandard2.0 and .NET Framework targets when LangVersion isn't set.

Failure scenario

1. A generator built on the toolkit calls WriteFileHeader(cb, copyright) and writes the rest of the file by hand, the way the PairGenerator fixture does. It emits only C# 7.3-compatible code.
2. A consumer project targets netstandard2.0 (or net48) and doesn't set LangVersion.
3. Before Enable nullable in every generated file's header [patch] #57 that output compiled. Since Enable nullable in every generated file's header [patch] #57 every generated file fails with CS8370, so the consumer's build is broken.

I reproduced this by running such a generator through CSharpGeneratorDriver with new CSharpParseOptions(LanguageVersion.CSharp7_3). The result was CS8370 at Things.g.cs(3,2).

WriteSourceFile output already needs C# 10, because CodeBlocker's SourceFileTemplate emits a file-scoped namespace. So the regression only hits the WriteFileHeader + hand-written-body path, which is exactly the path generator authors use when they need to support older consumers.

Suggested fix

Emit #nullable enable only when the consuming compilation's language version is 8 or later. For example, GeneratorBase.Initialize can combine context.ParseOptionsProvider into the pipeline and pass ((CSharpParseOptions)options).LanguageVersion through to WriteFileHeader / WriteSourceFile. That could be an overload, or a protected property set before Generate runs.

If supporting C# < 8 consumers is out of scope, document a minimum consumer language version in the README instead (the WriteFileHeader row, README.md:258). That way the break is a documented contract rather than a surprise.

Acceptance criteria

• A generator that uses WriteFileHeader + C# 7.3-compatible body compiles in a C# 7.3 consumer, with a test covering it in GeneratorHarness.
• Files generated for C# 8+ consumers still carry #nullable enable (Code written by WriteSourceFile has nullable disabled: every string? it emits raises CS8669, and non-annotated members are null-oblivious to consumers #55 stays fixed).