Contracts: Add a test suite for stable_swap_pool

[attyolu]

##closes #1220

Summary

Add a focused regression suite for the stable-swap pool to exercise the invariants and edge cases that matter for AMM safety.

What changed

• Added invariant coverage for add-liquidity, swap, and remove-liquidity sequences
• Asserted rounding behavior always favors the pool over the caller
• Verified slippage and minimum-output checks reject unsafe trades
• Covered extreme imbalance and near-empty pool states without panics or overflow
• Confirmed fee accounting is conserved across a multi-swap sequence
• Kept reentrancy regression coverage in place

Why

Stable-swap arithmetic is precision-sensitive. A small rounding or fee-accounting error can produce economic drift or exploitable behavior. These tests guard against the exact class of mistakes that are difficult to spot in review.

Verification

• Ran the stable-swap contract test suite
• Result: 6 passed, 0 failed

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