Refactor shared path normalization - #82

Summary

• centralize only the string-level path joining and separator normalization shared by Bash and PowerShell
• preserve shell-specific root, drive, provider, and resolver failure semantics
• add focused tests for the shared normalization contract
• advance the v0.3 OpenSpec implementation plan

Verification

• dotnet build -c Release
• dotnet test -c Release (1,132 passed)
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• dotnet format --verify-no-changes
• openspec validate v0-3-structured-shell-analysis --strict
• Slopwatch changed-file scan: 0 issues

Review

The rebased patch has the same stable patch ID as the previously adversarially reviewed change.