ConcurrentModificationException somewhere in the index

Happened couple of times recently: http://65.21.4.137:8080/app/1/bug/199/report

```
java.util.ConcurrentModificationException
at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:760)
at java.util.LinkedHashMap$LinkedValueIterator.next(LinkedHashMap.java:788)
at kotlin.collections.CollectionsKt___CollectionsKt.toCollection(_Collections.kt:1295)
at kotlin.collections.CollectionsKt___CollectionsKt.toSet(_Collections.kt:1348)
at space.taran.arklib.domain.index.PlainResourcesIndex.listResources(PlainResourcesIndex.kt:64)
at space.taran.arklib.domain.index.AggregatedResourcesIndex.listResources(AggregatedResourcesIndex.kt:26)
at space.taran.arklib.domain.index.ResourcesIndex$DefaultImpls.listIds(ResourcesIndex.kt:14)
at space.taran.arklib.domain.index.AggregatedResourcesIndex.listIds(AggregatedResourcesIndex.kt:21)
at space.taran.arklib.domain.index.ResourcesIndex$DefaultImpls.listAllIds(ResourcesIndex.kt:18)
at space.taran.arklib.domain.index.AggregatedResourcesIndex.listAllIds(AggregatedResourcesIndex.kt:21)
at space.taran.arknavigator.mvp.model.repo.stats.PlainStatsStorage.belongToRoot(PlainStatsStorage.kt:86)
at space.taran.arknavigator.mvp.model.repo.stats.PlainStatsStorage.access$belongToRoot(PlainStatsStorage.kt:26)
at space.taran.arknavigator.mvp.model.repo.stats.PlainStatsStorage$handleEvent$2.invokeSuspend(PlainStatsStorage.kt:70)
```

Start with PlainResourcesIndex.kt at line 64, then trace how AggregatedResourcesIndex.listResources and PlainStatsStorage.handleEvent reach it. Reproduce or inspect the reported index access path and confirm that resource listing remains correct without the ConcurrentModificationException during concurrent updates.

Written by the indexing model from the issue text.