fix: use checked arithmetic in platform fee calculation

[DTriple-p]

Closes: #545

Summary

This PR hardens platform fee arithmetic in contracts/predictify-hybrid/src/fees.rs so basis-point fee math cannot overflow before division.

What changed

• Added checked arithmetic helpers for fee math in FeeCalculator
• Replaced raw multiply-then-divide paths with checked operations in:
  • FeeCalculator::calculate_platform_fee
  • FeeCalculator::calculate_user_payout_after_fees
  • FeeCalculator::calculate_fee_breakdown
  • dynamic fee multiplier paths
  • fee withdrawal cap basis-point math
  • fee total accumulation in FeeTracker
• Added a dedicated typed error: Error::FeeArithmeticOverflow
• Documented rounding direction as floor rounding and reconciliation guarantees
• Added focused regression tests for:
  • zero pool rejection
  • 1-bps floor rounding
  • overflow-adjacent i128::MAX stake pools
  • fee breakdown reconciliation
  • no mutation when fee collection overflows
  • fee vault accumulation overflow rejection

Security Notes

Overflow safety

All fee multiplication/division paths now use checked arithmetic and return Error::FeeArithmeticOverflow instead of wrapping.

Rounding direction

Fee calculations round down using integer division. Because inputs are non-negative, this is equivalent to floor rounding.

This ensures:

• no accidental over-collection
• deterministic fee behavior
• exact reconciliation in fee breakdowns

platform_fee + user_payout_amount == total_staked

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