Refactor native argument fragment classification

[Aaronontheweb]

Summary

• extract one shell-neutral native argument fragment classifier
• preserve Bash and PowerShell provenance through explicit adapters
• remove duplicated native flag scanning without changing compatibility results
• complete OpenSpec preparation task 2.3

Security

The classifier consumes lexer-proved fragments and never rescans decoded text to infer expansion. Missing provenance remains opaque and fails closed.

Validation

• dotnet build -c Release --no-restore
• dotnet test -c Release --no-build --no-restore (1,125 passed)
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• dotnet format --verify-no-changes --no-restore
• openspec validate v0-3-structured-shell-analysis --strict
• Slopwatch on every changed C# file (zero findings)
• rebased patch ID matches the adversarially reviewed change exactly