Add direct PowerShell execution regions

[Aaronontheweb]

Summary

• parse direct & { ... } and . { ... } blocks as typed synchronous execution regions without synthetic host commands
• preserve dot-source state, isolate ordinary direct-call state, propagate shared location outcomes, and fail closed on explicit/dynamic parent-scope escapes
• promote the direct-call corpus case and teach the PowerShell corpus generator to serialize execution-region structure
• keep block arguments, param() declarations, and ordinary assignment interiors atomic pending the remaining task 7.5b grammar

Security review

• adversarial review: GO on frozen staged diff a5e0c30566ee898d2db65b60937de51f62d2cb3b97d9a6facf6e9b93278fc332
• live PowerShell 7.6.4 probes cover local/private/global/script/numeric scopes, environment and function providers, provider mutator differences, dynamic loop-derived names, and nested child-host isolation

Validation

• dotnet build -c Release --no-restore
• dotnet test -c Release --no-build (2,087 passed)
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• openspec validate v0-3-structured-shell-analysis --strict
• slopwatch analyze -d . --no-baseline --fail-on warning
• PowerShell corpus manifest regeneration: no semantic drift outside the promoted direct-call case