Add host-selected PowerShell dialect analysis

[Aaronontheweb]

Outcome

Adds the host-selected grammar boundary and explicit PowerShell dialect contract required for native Windows execution without cross-language parsing.

• keeps Bash and PowerShell as separate top-level grammars
• adds the compatible PowerShell7 default and explicit WindowsPowerShell51 fallback
• bounds the PowerShell 7 metadata contract to >=7.6.4 and <7.7
• rejects PowerShell 7 pipeline chains throughout every nested Windows PowerShell 5.1 parse path
• uses dialect-local aliases, receiver metadata, persistent state, and static child-host selection
• routes generated corpus and live oracle validation through the selected executable
• verifies powershell.exe from the same Bash PATH used by dotnet test on Windows CI
• documents executor, parser, approval-policy, and model-context identity requirements

Compatibility and security

The public API diff against published 0.3.0-alpha.4 contains exactly two additive changes on both net8.0 and netstandard2.0: PwshDialect and one PwshParserOptions property. Unknown dialects, unsupported 5.1 syntax, ambiguous receivers, and unavailable metadata remain fail closed. Cross-language payloads remain ordinary arguments.

Validation

• dotnet build -c Release: clean, zero warnings
• dotnet test -c Release --no-build: 2,813 passed
• focused dialect, catalog, corpus, and live-oracle suite: 963 passed
• generated PowerShell corpus: exact 503-file match
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify: passed
• dotnet pack -c Release --no-build: passed
• openspec validate v0-3-structured-shell-analysis --strict: passed
• Slopwatch strict full-tree scan: 0 findings
• adversarial review: PASS, no remaining findings

Auto-merge will be enabled after the first Windows run proves the Windows PowerShell 5.1 oracle gate.