fix: make late-fee arithmetic overflow-safe (Closes #1126)

[s6pa1rta3n-lab]

Summary

Closes #1126

Makes late-fee arithmetic overflow-safe across all fee modes, installment boundaries, and configuration states. Ensures deterministic execution at maximum amount and installment limits, defines floor rounding for delinquency surcharge calculations, and guarantees neutral protocol behavior when configuration is cleared.

Acceptance Criteria Checklist

• Overflow returns a stable error before mutation: Handled via Uint128::checked_mul and crate::accrual::accrued_interest, returning ContractError::Overflow before any state transition or storage write can occur. Verified in contracts/creditra-credit/tests/late_fee_overflow.rs::overflow_protection.
• Zero and maximum boundaries are tested: Exhaustively covered zero missed installments across all configs, single installment at Uint128::MAX, max u64::MAX installments at unit amount, maximum safe product and single-step overflow boundary, and surcharge bps bounds (0, 10_000, 10_001, u32::MAX). Verified in contracts/creditra-credit/tests/late_fee_overflow.rs::boundary_conditions and unit tests in contracts/creditra-credit/src/penalties.rs.
• Switching fee modes cannot carry stale state: Verified rapid alternating state transitions (None -> Flat -> AprBased -> None -> Flat -> AprBased -> None) and verified raw storage key b"lfc" isolation across mode switches. Verified in contracts/creditra-credit/tests/late_fee_overflow.rs::state_transition_and_clearing.
• Golden tests cover all public fee variants: Table-driven golden vectors cover Flat variants, AprBased variants with floor rounding, cleared (None) variant, and extreme boundary vectors. Verified in contracts/creditra-credit/tests/late_fee_overflow.rs::golden_tests.

Implementation Details & Code/Test Mapping

Acceptance Criterion	Implementation Symbol	Test Verification
Overflow Protection	compute_late_fee, compute_apr_late_fee	late_fee_overflow.rs::overflow_protection
Boundary Verification	validate_late_fee_config, compute_late_fee	late_fee_overflow.rs::boundary_conditions
Mode Isolation	execute_set_late_fee_config, compute_late_fee_optional	late_fee_overflow.rs::state_transition_and_clearing
Public Variants Golden Suite	LateFeeConfig	late_fee_overflow.rs::golden_tests
Authorization Boundaries	execute_set_late_fee_config	late_fee_overflow.rs::authorization_boundaries

Security & Failure-Mode Considerations

• Deterministic Failure: Arithmetic uses checked operations (checked_mul, checked_div). Any calculation that would exceed Uint128::MAX immediately yields ContractError::Overflow, halting execution before any state or balance modification takes effect.
• Rounding Policy: APR delinquency surcharge calculations apply integer floor division (truncation toward zero), matching protocol interest accrual policy and favoring borrowers. Flat fees use discrete token units with whole installment multipliers without fractional truncation.
• State Neutrality: When configuration is cleared (None), compute_late_fee_optional evaluates to zero, ensuring no penalty is charged and preventing stale parameters from affecting subsequent contract interactions.
• Authorization Enforcement: Only the contract admin can set or clear late-fee configuration; unauthorized attempts are rejected with ContractError::Unauthorized.

Verification

• cargo fmt -- --check: PASSED
• cargo clippy --all-targets -- -D warnings: PASSED
• cargo test --no-fail-fast: PASSED (all unit, integration, and property tests)
• cargo build --release: PASSED
• cargo build --release --lib --target wasm32-unknown-unknown: PASSED (373 KB WASM, within 614,400-byte budget)
• scripts/validate_schemas.sh: PASSED (schemas up to date)

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC