Fix for aggregators in `evt` to index output with `evt_idx` instead of `ch_idx`

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (724655c) 22.62% compared to head (e5fddca) 22.68%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #551 +/- ## ========================================== + Coverage 22.62% 22.68% +0.05% ========================================== Files 42 42 Lines 8106 8112 +6 ========================================== + Hits 1834 1840 +6 Misses 6272 6272 ```

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