test: complete Bash v0.3 design corpus

[Aaronontheweb]

Summary

• promote every remaining stable Bash v0.3 design input into the executable corpus
• pin exact substitution, cwd-join, bounded-loop, descriptor, and heredoc projections
• retain while, conditional, and process-substitution cases as explicit future scope

Validation

• dotnet build -c Release
• dotnet test -c Release --no-build (2,830 passed)
• focused corpus, design, and PII tests (817 passed)
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• openspec validate v0-3-structured-shell-analysis --strict
• adversarial review: PASS