Expand tests and document Python API

[DennisLent]

Summary

• add game advance scheduling tests for mid-advance and boundary-time processing
• harden CLI parsing with numeric validation, negative cash handling, and targeted command coverage
• widen GUI unit tests for click handling and default input fields

Testing

• cargo test
• pytest

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https://chatgpt.com/codex/tasks/task_e_68a4a8e8e5a48320bcb09dca7242f831

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 36.53%. Comparing base (9fd7b3a) to head (ea0cd9f).
⚠️ Report is 3 commits behind head on feat-python-api.

Additional details and impacted files

@@                 Coverage Diff                 @@
##           feat-python-api      #33      +/-   ##
===================================================
+ Coverage            33.80%   36.53%   +2.73%     
===================================================
  Files                   22       22              
  Lines                 1497     1497              
===================================================
+ Hits                   506      547      +41     
+ Misses                 991      950      -41     

Flag	Coverage Δ
rust	36.53% <ø> (+2.73%)	⬆️

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