Split load across nodes more evenly, useful for low tps configs

[marta-lokhova]

We've had this issue for a while, but this is a lot more relevant now that we're testing Soroban loads where tps is generally quite a bit lower than classic. the change spread the load more evenly across nodes

[Copilot]

Pull request overview

This PR adjusts how RunMultiLoadgen splits per-node loadgen settings so that “remainder” TPS (and related mixed-mode rates) is distributed across the first r nodes instead of being concentrated on a single node. This better balances per-node load in low-TPS configurations, especially when total TPS is smaller than the number of loadgen nodes.

Changes:

• Change integer-splitting logic to distribute attr % n across the first r nodes (instead of putting all remainder on the last node).
• For mixed pregen modes, derive per-node txrate from the per-node classicTxRate/sorobanTxRate shares to keep the three rate fields consistent on each peer.

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