fix issue 378

[Bezarius]

issue description: #378

[neuecc]

Thank you.
I was planning to overhaul the Source Generator, but that ended up making me come to a standstill.
I will accept ongoing improvements and release them in the near future.

[Copilot]

Pull request overview

This PR updates the source generator to consistently gate use of the C# 11 scoped modifier on both the target framework and language version, preventing mismatches with base formatter signatures as described in issue #378.

Changes:

• Update all generator scopedRef decisions to require both context.IsNet7OrGreater and context.IsCSharp11OrGreater(), ensuring scoped ref is only emitted when compatible with the runtime and language.
• Simplify the fixedSize detection logic by replacing a manual loop-and-flag pattern with a single Members.All(...) boolean expression.

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[Copilot]

The new scopedRef condition (context.IsNet7OrGreater && context.IsCSharp11OrGreater()) fixes the reported mismatch between generated scoped parameters and the base formatter signatures, but there doesn't appear to be a regression test that covers this specific matrix (C# 11 language version with IsNet7OrGreater == false). It would be helpful to add a generator test that simulates this environment and asserts that the generated formatter methods use plain ref, so future changes to IsNet7OrGreater/language-version logic don't reintroduce this compiler error.