Add unit tests for the investment share percentage calculator handling floating point correctly using integer arithmetic

[Chucks1093]

Summary

Share percentages must be calculated using integer arithmetic to avoid floating-point drift (e.g. 1/3 = 33.333…%). Unit tests should confirm the calculator returns correct percentages, that all shares sum to exactly 100%, and that the remainder is assigned to the largest investor.

Scope

• Test three equal investors of 3000 each (total 9000) each get 33% with 1% remainder assigned to the first investor
• Test two investors with 2500 and 7500 get 25% and 75% exactly
• Test a single investor gets 100%
• Assert the sum of all returned percentages equals exactly 100
• Assert remainder always assigned to the investor with the largest commitment

Acceptance Criteria

• Equal three-way split sums to 100% with remainder assigned correctly
• 25/75 split returns exact percentages
• Single investor returns 100%
• All percentages always sum to exactly 100
• No floating-point arithmetic used in implementation

ETA: 12 hours

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