Add transaction fee comparison across networks

[Just-Bamford]

Problem

Fees vary across networks (mainnet vs testnet). No way to compare fees between networks.

Solution

Implement \compareFeeAcrossNetworks(transaction, networks[])\ in \src/transaction/index.ts. Returns fee comparison for each network.

Acceptance Criteria

• Function added to \src/transaction/index.ts\
• Accepts networks array
• Returns fees for each network
• Tests cover multiple networks

Note for contributors

Simulate on each network. Reference fee estimation. Add tests to \src/tests/transaction.test.ts.