Test/remit split schedule id sequencing

[Agbasimere]

Closes: #250

Description

This PR implements replay-safe schedule ID sequencing for the RemittanceSplit contract. It ensures that every remittance schedule is assigned a strictly monotonic, unique identifier, preventing ID collisions even across high-volume creation and modification cycles.

Key Changes

• Monotonic ID Counter: Implemented a synchronized NEXT_RSCH storage counter to generate unique IDs.
• Safe Arithmetic: Integrated checked_add to prevent u32 overflow vulnerabilities, returning RemittanceSplitError::Overflow if the limit is reached.
• Collision Protection: Added an explicit storage-level check (contains_key) before assignment as a "defense-in-depth" measure for ID uniqueness.
• NatSpec Documentation: Added formal documentation to the creation logic in remittance_split/src/lib.rs defining the security invariants.
• Security Updates: Updated remittance_split/README.md to document the schedule ID sequencing as a core platform invariant.

Verification & Testing

Unit & Stress Tests

Added a suite of validation tests in remittance_split/tests/stress_test_large_amounts.rs:

• test_schedule_id_sequencing_monotonicity: Verifies steady incrementing (1, 2, 3...) for 100+ operations.
• test_schedule_id_uniqueness_across_operations: Confirms IDs remain unique and locked during modification and cancellation.
• test_high_volume_schedule_creation_no_collisions: Bulk-creates 500 schedules and verifies 100% uniqueness via key mapping.

Security Invariants

• Monotonicity: IDs only increase, ensuring the history cannot be "rolled back" or replayed under an old ID.
• Uniqueness: IDs are never reused, even after a schedule is cancelled.