perf: use shared channel for prewarm workers

[klkvr]

Changes prewarming to use single mpmc channel instead of a channel per worker. This allows for better utilization towards the end vs right now prewarming will be blocked on the slowest worker pushing through their txs

## Configuration
- **Blocks**: 2000

## Commits
- **Baseline**: c7faafd (main)
- **Feature**: 47519b1 (klkvr/prewarm-utilization)

## Results

| Metric | Baseline | Feature | Change |
|--------|----------|---------|--------|
| Mean Latency | 37.83ms | 37.42ms | -1.09% ≈ |
| Std Dev | 22.61ms | 22.18ms | |
| P50 Latency | 29.73ms | 29.22ms | -1.71% ≈ |
| P90 Latency | 71.23ms | 69.9ms | -1.87% |
| P99 Latency | 117.26ms | 115.29ms | -1.68% |
| Gas/sec | 799.83 Mgas/s | 808.62 Mgas/s | 1.1% ≈ |