research: Canton network fee module

## Summary

The middleware currently charges **no network fee**, but the Canton participant node still burns **Canton Coin (CC)** to pay the synchronizer for traffic on every transaction it submits. The operator absorbs 100% of that cost. This issue tracks the research and design for a **fee module** that distributes the node's traffic cost to the end users who cause it, with a strict **no-loss** guarantee for the operator (exact cost-recovery; margin optional).