Specify typed PowerShell execution regions

[Aaronontheweb]

Summary

• lock the additive v0.3 ExecutionRegionSyntax API with typed origin, phase, timing, and cardinality
• distinguish direct call, dot-source, and command-argument regions without source reparsing
• pin the PowerShell 7 receiver/state matrix, including the valid Invoke-Command parameter-set boundary
• expand the design corpus with synchronous, concurrent, deferred, data, unknown-receiver, and atomic fail-closed cases
• keep parser emission for subsequent small implementation PRs

Security behavior

Unknown receivers remain conservative, unsupported interiors fail atomically, and direct block arguments or leading param declarations remain unparseable until their binding grammar is modeled completely.

Validation

• dotnet build -c Release --no-restore
• dotnet test -c Release --no-build: 1,916 passed
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• openspec validate v0-3-structured-shell-analysis --strict
• slopwatch analyze --no-baseline --fail-on warning: 0 findings
• dotnet pack -c Release --no-build -o ./bin/nuget
• adversarial review of exact diff 8177ca88: GO, no blocking findings