fix(swap): fall back to 1 not 0 on exponent overflow

[loom-agent]

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Release Notes

• Fixed a precision-overflow edge case in the Balancer swap exponent calculation where returning 0 instead of 1 could pay out the entire reserve for a trivial input.

Problem

Balancer::exp_scaled computes (x / (x + dx))^w, which must stay in (0, 1]. When the intermediate U64F64 division loses precision to zero, the fallback returns 0. Plugging e=0 into the Balancer formula gives (1-e) = 1, causing the swap to pay out the entire reserve for a trivial input amount.

Root Cause

In pallets/swap/src/pallet/balancer.rs, the fallback value at the end of exp_scaled is U64F64::saturating_from_num(0). The Balancer formula amount_out = reserve * (1 - e) treats e as the exponent; e=0 means 100% payout.

The Fix

Change the fallback from 0 to 1. When e=1, the swap produces zero output and fails safely via the InsufficientLiquidity check (which was also tightened from < to <= to catch any remaining edge).

Testing

Added audit_probe_exp_scaled_stays_within_unit_interval which fuzzes exponent calculations across weight values, reserve sizes, and delta magnitudes, asserting that every result stays <= 1.

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

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