logic to switch math based on the curve type.

[BigJohn-dev]

Closes #6

Summary

Adds a VestingCurve enum to the Vesting Vault contract, enabling Linear (proportional) and Exponential (slow-start, fast-finish) vesting schedules. The curve is immutable after initialization, with updated logic in vested_amount, claim, and status, plus full unit and integration test coverage.

Key Updates

• Linear: total × elapsed ÷ duration
• Exponential: total × elapsed² ÷ duration²
• Curve set at initialize() and cannot change
• Incremental claims handled correctly
• 11 tests covering math and behavior

Testing

Run cargo test (all 11 should pass), build with stellar contract build, deploy, initialize with either curve, and verify 50% elapsed returns 50% (Linear) or 25% (Exponential).

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