feat: add Monte Carlo bankroll simulation

Simulate N bankroll trajectories given a set of bets to visualize risk of ruin:

```ts
monteCarloSim({
  bets: 500,
  winRate: 0.55,
  avgOdds: -110,
  kellyFraction: 0.5,
  startingBankroll: 1000,
  simulations: 10000
})
// { medianFinal: 1823, riskOfRuin: 0.02, worstCase: 312, bestCase: 8442 }
```