fix(bench): round-robin RPC sends and limit report concurrency

[decofe]

bench send now dispatches transactions round-robin across every supplied RPC endpoint, giving the multi-validator benchmark an even submission distribution. Limit post-workload block-stat fetches to four concurrent requests (previously 32) to reduce RPC-tunnel pressure during report collection; transaction concurrency and the measured workload window are unchanged.

Validated with all 114 bench-core tests, nightly formatting and Clippy, and the full GitHub checks. Multi-region benchmark attempt 3 completed both phases and report collection with exit code 0: 10 validators across five AWS regions, 100 GiB state, 90-second load windows, 50,000 target TPS, and 5,000 concurrent submissions. Both sides used this txgen revision and Tempo d093829e40704cf2472a3ef18d4a85695e6ec71f; all ten feature processes were verified to include --disable-tx-gossip, and all ten baseline argument files omitted it.

Gossip on → off in this single pair: on-chain TPS 8,240 → 7,928 (−3.8%), RPC submission failures 1,609 → 0, builder pool-fetch p50 7.2 → 0.8 ms, and block p99 1,427 → 1,468 ms. The first two attempts timed out during baseline block-stat collection; the original reference run had also encountered that failure before this change.

Prompted by: @mediocregopher