feat: add overflow-safe score and probability arithmetic

[SupremeCarMart100]

Summary

Replaces raw integer arithmetic in score and probability calculation paths with checked or saturating
operations, eliminating the risk of silent wrapping or panics when adversarial or extreme stake values
are passed through the payout pipeline.

Adds a new ArithmeticUtils struct as a single source of truth for checked math primitives used across the
contract.

Changes

New: ArithmeticUtils in utils.rs

Four #[inline] primitives that return Err(Error::Overflow) (code 672) instead of panicking or silently
wrapping:

• checked_add — rejects negative operands, errors on overflow
• checked_mul — same contract
• checked_mul_div(value, numerator, denominator) — guards intermediate product and division by zero
• checked_accumulate — drop-in replacement for += in loops over user-controlled values

Fixed call sites

┌────────────┬───────────────────────────────┬────────────────────────────────────────────────────────┐
│ File │ Function │ Change │
├────────────┼───────────────────────────────┼────────────────────────────────────────────────────────┤
│ bets.rs │ calculate_implied_probability │ outcome * 100 → checked_mul_div, safe fallback 0 │
├────────────┼───────────────────────────────┼────────────────────────────────────────────────────────┤
│ bets.rs │ calculate_payout_multiplier │ total * 100 → checked_mul_div, safe fallback 0 │
├────────────┼─────────────────────────────────┼──────────────────────────────────────────────────────┤
│ queries.rs │ calculate_payout │ stake * total_staked, share * 2 → checked_mul_div, │
│ │ │ propagates Error::Overflow │
├────────────┼─────────────────────────────────┼──────────────────────────────────────────────────────┤
│ queries.rs │ calculate_outcome_pool │ pool += stake → checked_accumulate, propagates │
│ │ │ Error::Overflow │
├────────────┼─────────────────────────────────┼──────────────────────────────────────────────────────┤
│ queries.rs │ calculate_implied_probabilities │ pool * 100, pool1 + pool2 → checked ops, safe │
│ │ │ fallback 50 │
├────────────┼─────────────────────────────────┼──────────────────────────────────────────────────────┤
│ voting.rs │ calculate_user_payout │ winning_total += → checked_accumulate, propagates │
│ │ │ Error::Overflow │
├────────────┼─────────────────────────────────┼──────────────────────────────────────────────────────┤
│ markets.rs │ calculate_payout │ already used checked_mul but mapped to InvalidInput; │
│ │ │ upgraded to Error::Overflow for precise diagnostics │
├────────────┼─────────────────────────────────┼──────────────────────────────────────────────────────┤
│ markets.rs │ calculate_winning_stats │ winning_total += → saturating_add │
├────────────┼─────────────────────────────────┼──────────────────────────────────────────────────────┤
│ utils.rs │ calculate_percentage │ raw * → saturating_mul; added zero-denominator guard │
├────────────┼─────────────────────────────────┼──────────────────────────────────────────────────────┤
│ utils.rs │ weighted_average │ raw * / += → saturating_mul / saturating_add │
├────────────┼─────────────────────────────────┼──────────────────────────────────────────────────────┤
│ utils.rs │ simple_interest │ chained raw * → saturating_mul chain │
└────────────┴─────────────────────────────────┴──────────────────────────────────────────────────────┘

Tests

New file: src/arithmetic_overflow_tests.rs — 36 tests across three modules:

• arithmetic_utils_tests — checked_add, checked_mul, checked_mul_div, checked_accumulate: normal,
  i128::MAX boundary, overflow, negative-input rejection, zero-denominator, idempotency
• numeric_utils_overflow_tests — saturation behavior for calculate_percentage, weighted_average,
  simple_interest
• market_utils_payout_tests — calculate_payout: normal, zero stake, zero winning total, overflow on
  stake×fee and share×pool, proportional split, idempotency

test result: ok. 36 passed; 0 failed
Full suite: 703 passed (up from 701); 147 pre-existing failures unchanged

Compatibility

• All public function signatures are unchanged
• Error::Overflow (code 672) already existed in the enum — no new error codes added
• Callers that previously received a panic or silent wrap will now receive Error::Overflow; callers of
  infallible helpers (calculate_percentage, etc.) see saturating results rather than wrapping

Security & failure modes

• Overflow is now explicit and diagnosable via the Error::Overflow code rather than silent data
  corruption
• All fixed functions are pure — no mutable state — so retries and concurrent calls cannot produce
  inconsistent results
• Negative financial inputs are rejected at the ArithmeticUtils boundary, enforcing the contract
  invariant that stakes and totals are always non-negative

closes #1410

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