Add late-payment penalty surcharge for delinquent credit lines

[Topmatrixmor2014]

Closes #355

Summary

Implements an admin-configurable penalty surcharge (in basis points) that applies on top of the base \interest_rate_bps\ when a credit line is delinquent. The penalty serves as a deterrent for late payments and encourages timely repayment.

Implementation Details

• Storage: Added \PenaltySurchargeBps\ enum variant to store admin-configurable penalty rate
• Risk Module: Added \set_penalty_surcharge_bps\ and \get_penalty_surcharge_bps\ public functions with admin auth and bounded validation
• Accrual Logic: Modified \�pply_accrual\ to check delinquency status and apply penalty surcharge when applicable
• Events: Added \PenaltyRateEnteredEvent\ and \PenaltyRateExitedEvent\ for tracking rate transitions
• Contract API: Exposed penalty surcharge functions in lib.rs
• Tests: Created comprehensive test suite in \penalty_surcharge.rs\ covering all edge cases

Acceptance Criteria (from #355)

• Surcharge applied only while delinquent
• Effective rate clamped to \MAX_INTEREST_RATE_BPS\
• Enter/exit penalty events emitted

Test Coverage

Created comprehensive test suite covering:

• Setting/getting penalty surcharge
• Default value validation (0)
• Maximum rate enforcement (cannot exceed 10_000 bps)
• Application to delinquent vs. non-delinquent lines
• Event emission for penalty rate transitions
• Clamping to max rate when base + surcharge exceeds limit
• Zero surcharge behavior (no effect)

Notes

• Overflow-safe clamp math implemented using \saturating_add\ and \min()\
• Effective rate computed as \�ase_rate.saturating_add(penalty_surcharge).min(MAX_INTEREST_RATE_BPS)\
• Penalty only applies when \is_delinquent\ returns true
• Default penalty surcharge is 0 (disabled)
• All functions include inline documentation

Generated with Devin (https://cli.devin.ai/docs)

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