Test/vault max balance closes #264

[ChukwuemekaP1]

Summary

Added explicit max-balance boundary tests across the Callora contracts to ensure overflow safety. These tests verify that the system fails safely and deterministically when an operation would push a balance beyond the i128::MAX limit.

Changes

Vault Contract

• Added deposit_max_balance_overflow_panic: Exercises the checked_add logic in deposit() by setting the balance to i128::MAX - 1 and attempting a 2-unit deposit.

Revenue Pool Contract

• Added batch_distribute_max_balance_overflow_panic: Confirms that calculating the total sum for batch distributions panics safely if it exceeds i128::MAX.

Settlement Contract

• Added test_pool_balance_overflow_panic: Verifies the global pool cannot overflow.
• Added test_developer_balance_overflow_panic: Verifies individual developer balances cannot overflow.

Documentation

• Updated INVARIANTS.md to explicitly define the upper-bound balance behavior (<= i128::MAX).

Verification

• Unit Tests: All 250+ tests (including the 4 new boundary tests) passed.
• Linter: cargo clippy passed with no warnings.
• Build: Successfully compiled callora-vault to WASM release target.
• Safety: Confirmed that checked_add(...).unwrap() paths correctly trigger transaction reverts on overflow rather than silent wrapping.

closes #264

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