Use tarpaulin config and test error displays

[DennisLent]

Summary

• configure tarpaulin to skip GUI, Python, and CLI entrypoint files
• add comprehensive display tests for GameError variants
• point CI coverage job at shared config without redundant crate exclusion

Testing

• cargo fmt --all
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test -p rusty_runways_core
• cargo tarpaulin --workspace --out Xml --skip-clean

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https://chatgpt.com/codex/tasks/task_e_68a5f61fc4288320b663cbcfbba24548

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.63%. Comparing base (1edeca2) to head (7f744dd).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #35       +/-   ##
===========================================
+ Coverage   36.53%   60.63%   +24.09%     
===========================================
  Files          22       17        -5     
  Lines        1497      912      -585     
===========================================
+ Hits          547      553        +6     
+ Misses        950      359      -591     

Flag	Coverage Δ
rust	60.63% <ø> (+24.09%)	⬆️

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