Harden v0.3 public API compatibility tests

[Aaronontheweb]

Summary

• pin exact legacy and v0.3 public property sets, reference nullability, parser entry points, and exported types against the synchronized specs
• test generated ParsedCommand equality, equal-record hash consistency, and ToString() participation for Syntax and Commands
• prove default JSON is not a polymorphic parser-result round trip and that the library has no JSON annotations or dependency
• make every policy-sensitive future enum value detectable for fail-closed consumer handling
• close OpenSpec verification tasks 11.1 and 11.5 with executable evidence

Validation

• API contract tests: 40/40 on .NET 10 and independently 40/40 on .NET 8
• dotnet build -c Release (0 warnings)
• dotnet test -c Release --no-build (2,373 passed)
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• openspec validate v0-3-structured-shell-analysis
• Slopwatch strict scan of changed C# files: 0 issues
• adversarial compatibility review: PASS