🧪 test: test getXPForLevel boundary values

[marcin2121]

🎯 What: The testing gap addressed
The getXPForLevel mathematical logic lacked testing, which is needed to prevent regressions when balancing or changing formulas.

📊 Coverage: What scenarios are now tested

• Valid normal levels (level 1, level 2, level 3)
• Boundary checks for level <= 1 (e.g. 0, negatives)
• Falsy/Invalid numeric scenarios (e.g. NaN) correctly bounded to level 1 returning BASE_XP.

✨ Result: The improvement in test coverage
Installed Vitest into devDependencies and added engine.test.ts to fully cover the mathematical behaviour of getXPForLevel.

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PR created automatically by Jules for task 13262986717890080875 started by @marcin2121

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