🧹 [Refactor excessively long function register_file]

[Tcode-Motion]

🎯 What: The register_file function in stdlib/src/file.rs was excessively long and difficult to read because it contained five large inline closures defining file operations (read, write, copy, remove, exists). I have extracted these closures into standalone module-level functions (file_read, file_write, file_copy, file_remove, file_exists).

💡 Why: Breaking down this monolithic function improves maintainability, readability, and testability. It follows the codebase pattern of using named standalone functions for StdFnCallback signatures rather than embedding complex logic inside the registry macro/insert blocks.

✅ Verification: I confirmed that the extraction was functionally equivalent by running cargo test -p techscript_stdlib and then running the full workspace test suite cargo test --workspace. All tests passed successfully. I also ran cargo fmt --all to ensure formatting compliance.

✨ Result: The register_file function is now much smaller and serves only as a clear registration block, delegating the actual implementation to well-defined, standalone helper functions.

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PR created automatically by Jules for task 4472496660871607341 started by @Tcode-Motion

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