Record alpha.3 publication

[Aaronontheweb]

Summary

• Mark ShellSyntaxTree 0.3.0-alpha.3 as published.
• Link the live NuGet package and GitHub prerelease.
• Record that the package preserves the v0.2 projection and public v0.3 API.

Validation

• dotnet build -c Release: passed with zero warnings
• dotnet test -c Release --no-build: 2,765 passed
• openspec validate --all --strict: passed
• header verification: passed
• Slopwatch with no baseline: zero findings
• git diff --check: passed
• adversarial review: PASS with no material findings