feat: add investment share percentage calculator and tests

[probablyABug]

Description

Closes #204

Adds unit test coverage for the investment share percentage calculator, verifying that percentage calculations use integer arithmetic and always produce a total of exactly 100%. The tests cover equal and unequal investment splits, single-investor cases, and correct allocation of rounding remainders to the largest investor.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✨ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📝 Documentation update
• 🎨 UI/UX improvement
• ♻️ Code refactoring
• ✅ Test addition or update
• 🔧 Configuration change

Checklist

• All GitHub Actions workflows are green on this PR (required for merge)
• Commit messages follow Conventional Commits (feat:, fix:, chore:, etc.) — enforced by CI
• No secrets, API keys, .env, or credentials committed (see CONTRIBUTING.md)
• My code follows the code style of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Testing

Added unit tests for the investment share percentage calculator covering equal splits, unequal splits, single-investor calculations, and rounding remainder allocation.

How to Test

1. Run the backend test suite.
2. Verify the three-way 3000/3000/3000 split produces percentages totaling exactly 100%, with the rounding remainder assigned to the first/largest investor.
3. Verify a 2500/7500 split produces exactly 25% and 75%.
4. Verify a single investor receives exactly 100%.
5. Verify all test cases sum to exactly 100% and do not rely on floating-point arithmetic.

Test Coverage

• Unit tests added/updated
• Integration tests added/updated
• E2E tests added/updated (if applicable)
• Manual testing completed

Screenshots (if applicable)

Not applicable — this PR adds backend unit test coverage only.

Additional Notes

The tests specifically guard against floating-point rounding drift in investment share calculations. They also verify that any percentage remainder is assigned consistently to the investor with the largest commitment.

For Reviewers

• Code quality and readability
• Test coverage
• Security implications
• Performance impact
• Breaking changes

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