Split game engine and CLI into workspace crates

[DennisLent]

Summary

• reorganize into a Cargo workspace with separate rusty_runways_core and rusty_runways_cli crates
• move all engine code to crates/core and re-export Game
• move CLI and readline code to crates/cli depending on the core crate
• document workspace layout and build/test instructions

Testing

• cargo build
• cargo test

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https://chatgpt.com/codex/tasks/task_e_688defb8731c8320b06deb1379243c7b

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.92%. Comparing base (ae1eae5) to head (ef4cc71).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
+ Coverage   54.15%   54.92%   +0.77%     
==========================================
  Files          15       18       +3     
  Lines         759      772      +13     
==========================================
+ Hits          411      424      +13     
  Misses        348      348              

Flag	Coverage Δ
rust	54.92% <ø> (+0.77%)	⬆️

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