Fix three code bugs

[d2kole]

Fixes three bugs related to package exports, lambda closure behavior, and mutable default arguments to align the codebase with expected functionality and pass all tests, while also adding project planning documentation.

The lambda_array fix intentionally restores Python's late-binding behavior for loop variables within closures, as existing tests and main.py explicitly assert against this specific behavior (all lambdas adding 9). The foo() fix addresses state leakage caused by mutable default arguments, ensuring that each function call operates on a fresh list, preventing unintended side effects across invocations. This PR also introduces .cursor/rules/.cursorrules, PLANNED_INTERFACE.md, IMPLEMENTATION_PLAN.md, and TODO.md to document project workflow and planning artifacts.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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