Add safe Invoke-Expression command-string recursion

[Aaronontheweb]

Summary

• recurse into provably static Invoke-Expression and iex payloads so security consumers see the inner clauses
• safe-fail dynamic, interpolated, pipeline-fed, ambiguous, escaped, and unsupported invocation shapes without changing the public API
• preserve current-scope PowerShell location semantics and share the existing input-size and recursion-depth limits
• expand the PowerShell corpus to 262 cases, including adversarial escape, Unicode, module-qualification, and cwd-attribution variants

Validation

• dotnet build -c Release
• dotnet test -c Release (929 passed)
• dotnet pack -c Release -o ./bin/nuget
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• openspec validate recurse-static-invoke-expression
• real PowerShell oracle and PII audit
• two independent adversarial reviews with no remaining actionable findings

Closes #63

[Aaronontheweb]

LGTM