Improve TypeScript type safety in netBalances with explicit type assertions

When spreading the reverse object in netBalances function, TypeScript may not guarantee that reverse matches type T since it comes from a find() that could theoretically return undefined (though the if-block guards it). This risks type safety issues if the generic constraint changes.

Consider explicitly typing or re-constructing the object with known T properties, e.g., result.push({ ...reverse, amount: -diff } as T);

This was identified in Copilot AI review of PR #24.