Fix test data and comments in index calculation tests

[Copilot]

Test cases had incorrect expected values and misleading comments due to miscalculations of stars from experience and progression rates.

Issues Fixed

Incorrect index calculations:

• Line 1272: Expected index 16 with comment "4 stars" for 500 exp, but 500 exp = 1 star → index = 1 × 2² = 4
• Line 1330: Comment "4 stars" for 0 exp, corrected to "0 stars"
• Line 1272: Milestone adjusted from 20 to 10 to align with corrected index value

Incorrect formula coefficients:

• Lines 1622, 1654: Formula used 0.1*t for stars/day, but calculation shows 0.05 stars/day (2435 exp / 4870 avg exp per star / 10 days)

Misleading comments:

• Lines 1611, 1643: "0.5 stars per day" actually meant 0.5 total avg stars gained over 10 days (= 0.05 stars/day)

Variable naming:

• Line 1737: discriminants → discriminant (function returns single value)

Example

// Before: incorrect star calculation and milestone
expected: {
    index: 16, // 4 stars * (2 fkdr)^2
    milestone: 20,
}

// After: correct calculation
expected: {
    index: 4, // 1 star * (2 fkdr)^2
    milestone: 10,
}

Original prompt

Check for any issues in the index calculation tests and fix them. The tests are verified working, but some of them may have minor issues. Check the test cases and the test implementation. Note that the code is not implemented so the tests will fail, just look for issues in the tests without running them

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[Copilot]

Pull request overview

This PR fixes incorrect test data and misleading comments in index calculation tests. The changes correct mathematical errors in star-to-experience conversions and progression rate calculations.

Changes:

• Corrected index calculation for 500 exp (1 star, not 4 stars) with adjusted milestone
• Fixed comment describing 0 exp as "0 stars" instead of "4 stars"
• Corrected formula coefficients from 0.1 to 0.05 stars/day in two test cases
• Clarified comments to distinguish between total stars gained vs stars per day
• Fixed variable naming from plural discriminants to singular discriminant

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[Amund211]

Implemented back into #135