Large balance deduct

[clintjeff2]

test: large balance and large deduct

Summary

Adds unit tests that exercise vault arithmetic at the boundaries of i128 to ensure no overflow or underflow occurs with large numbers.

Changes

• contracts/vault/src/test.rs — 7 new test cases under the "large balance and large deduct" section:

Test	What it covers
large_balance_init_and_deduct	Init with i128::MAX / 2, deduct i128::MAX / 4, assert remaining
large_balance_deduct_entire_balance	Init with i128::MAX, deduct full amount, assert zero
large_balance_sequential_deducts	Init with 1e18, two sequential deducts draining to zero
large_batch_deduct_correctness	Batch deduct three equal large chunks, verify remainder
deposit_overflow_panics	Init near i128::MAX, attempt deposit that would overflow — expects failure
large_deduct_exceeding_balance_fails	Deduct more than a large balance — expects failure, balance unchanged

How it was tested

All tests pass locally:

cargo test --package callora-vault

###TEST Proof

###Build Proof

Checklist

• Tests pass with cargo test
• No new warnings
• Covers overflow and underflow edge cases
• Follows existing test style and conventions

Closes #32