fix: resolve duplicate exports and case sensitivity issues

[imjuni]

Summary

• Fix duplicate export statements in create mode processing
• Resolve case sensitivity issues on macOS/Windows filesystems
• Add comprehensive test coverage for getCorrectCasedPath function

Changes Made

1. Duplicate Export Fix: Removed incorrect filenamify() usage from export statement generation
2. Case Sensitivity Fix: Implemented getCorrectCasedPath() function to handle filesystem case mismatches
3. Code Quality: Enhanced with orThrow, atOrThrow for safer array/object access
4. Test Coverage: Added 100% statement coverage tests with proper mocking
5. Performance: Optimized getSep() usage to call once per function execution

Test Results

• ✅ All existing tests pass
• ✅ 100% statement coverage achieved
• ✅ Windows/Unix path handling properly tested

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (bd8792f) to head (5fe806e).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #187   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           88        90    +2     
  Lines         2185      2234   +49     
  Branches       424       441   +17     
=========================================
+ Hits          2185      2234   +49     

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