Implement Bash loop effective cwd transfers

[Aaronontheweb]

Summary

• interpret every admitted cd / chdir from ordered, visit-local effective argv
• model invalid and multiple operands as failure-only, recursively prove command / builtin wrappers, and preserve query semantics
• sanitize stale compatibility path resolutions and reject post-loop binding mutation or tracked unknown argv arity
• expand unit, native Bash-oracle, design-corpus, and executable-corpus coverage
• complete OpenSpec task 6.5c.3 and update the implementation plan

Validation

• dotnet build -c Release --no-restore
• dotnet test -c Release --no-build (1,666 passed)
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• dotnet format --verify-no-changes --no-restore
• openspec validate v0-3-structured-shell-analysis --strict --no-interactive
• Slopwatch on all modified C# files: 0 findings
• adversarial exact-stage review: GO for da2bc1ff1bb44d579cbca19f970de9c4acfe838fa64574bc674e843809932418