feat: build the analyzer against four Roslyn API versions

[samtrion]

Summary

• Splits NetEvolve.Analyzer into per-Roslyn-version build projects (4.4.0, 4.7.0, 4.14.0, 5.6.0 — latest), each packed into its own analyzers/dotnet/roslynX.Y/cs folder so .NET SDKs 8.0.400+ automatically load the highest version they support. NetEvolve.Analyzer.csproj itself becomes a pure packing project with no source of its own.
• Test projects (NetEvolve.Analyzer.Tests.Unit/.Integration) now build against multiple target frameworks (net6.0-net10.0, plus net472/net48/net481 on Windows) and reference a different Roslyn variant per framework, so the same test suite actually exercises several Roslyn APIs in one run instead of only one.
• Mutation testing is split into per-variant stryker-config.roslyn*.json files, since Stryker.NET has no way to target multiple source projects from a single configuration; the mutation workflow now runs them as a matrix.
• Adds classic .NET Framework compatibility for the test harness: a Polyfill package reference (with TUnit's own conflicting auto-injection disabled), and a shared FrameworkReferences helper per test project providing a portable fallback for resolving framework reference assemblies when TRUSTED_PLATFORM_ASSEMBLIES isn't populated (net472/net48/net481).
• Fixes a couple of latent CA2016/CA1307 issues that the new Roslyn variants and target frameworks surfaced.

Test plan

• dotnet build Analyzer.slnx -c Release — all 5 src projects (pack + 4 Roslyn variants) and both test projects build across all 8 target frameworks with 0 warnings/errors
• dotnet test Analyzer.slnx -c Release — 1504/1504 tests pass across all target frameworks
• csharpier check . — clean
• Verified the packed nupkg contains the expected analyzers/dotnet/roslyn{4.4,4.7,4.14,5.6}/cs/NetEvolve.Analyzer.dll layout and that a consumer project on each locally installed SDK (6.0.428, 8.0.423, 9.0.316, 10.0.302) loads exactly one matching variant, no duplicates

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.88%. Comparing base (07c7093) to head (054dd7b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #30   +/-   ##
=======================================
  Coverage   95.88%   95.88%           
=======================================
  Files          19       19           
  Lines         777      777           
  Branches      141      141           
=======================================
  Hits          745      745           
  Misses         12       12           
  Partials       20       20           

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