Add contribution amount calculator utility

Create a utility to calculate total pot size, per-member cost, and round projections.

**Acceptance Criteria:**
- Add `calculatePotSize(contributionAmount, memberCount)` function
- Add `calculateTotalCost(contributionAmount, totalRounds)` function
- Add `calculateExpectedPayout(contributionAmount, memberCount)` function
- Include in SDK exports

**Files:** `src/utils.ts`, `src/index.ts`