Half-written metadata

We need atomic writing for metadata and other computed values.
- https://github.com/ARK-Builders/arklib/issues/48

```
kotlinx.serialization.json.internal.JsonDecodingException: Cannot begin reading element, unexpected token: 10 at path: $
JSON input:
at kotlinx.serialization.json.internal.JsonExceptionsKt.JsonDecodingException(JsonExceptions.kt:24)
at kotlinx.serialization.json.internal.JsonExceptionsKt.JsonDecodingException(JsonExceptions.kt:32)
at kotlinx.serialization.json.internal.AbstractJsonLexer.fail(AbstractJsonLexer.kt:528)
at kotlinx.serialization.json.internal.AbstractJsonLexer.fail$default(AbstractJsonLexer.kt:526)
at kotlinx.serialization.json.internal.JsonTreeReader.read(JsonTreeReader.kt:104)
at kotlinx.serialization.json.internal.StreamingJsonDecoder.decodeJsonElement(StreamingJsonDecoder.kt:33)
at kotlinx.serialization.json.JsonElementSerializer.deserialize(JsonElementSerializers.kt:51)
at kotlinx.serialization.json.JsonElementSerializer.deserialize(JsonElementSerializers.kt:27)
at kotlinx.serialization.json.internal.PolymorphicKt.decodeSerializableValuePolymorphic(Polymorphic.kt:59)
at kotlinx.serialization.json.internal.StreamingJsonDecoder.decodeSerializableValue(StreamingJsonDecoder.kt:38)
at kotlinx.serialization.json.Json.decodeFromString(Json.kt:100)
at kotlinx.serialization.json.Json.parseToJsonElement(Json.kt:128)
at space.taran.arklib.domain.meta.MetadataStorage.valueFromBinary(MetadataStorage.kt:35)
at space.taran.arklib.domain.storage.FolderStorage$readFromDisk$jobs$2$1.invokeSuspend(FolderStorage.kt:94)
at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
at kotlinx.coroutines.internal.LimitedDispatcher.run(LimitedDispatcher.kt:42)
at kotlinx.coroutines.scheduling.TaskImpl.run(Tasks.kt:95)
at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(CoroutineScheduler.kt:570)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(CoroutineScheduler.kt:750)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(CoroutineScheduler.kt:677)
at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(CoroutineScheduler.kt:664)
Suppressed: kotlinx.coroutines.DiagnosticCoroutineContextException: [StandaloneCoroutine{Cancelling}@8ef4f0d, Dispatchers.IO]
```

Start at MetadataStorage.valueFromBinary in MetadataStorage.kt:35 and FolderStorage.readFromDisk in FolderStorage.kt:94, then inspect the code that writes metadata and computed values. Reproduce or test an interrupted write and verify that atomic persistence prevents the shown JsonDecodingException while keeping metadata readable.

Written by the indexing model from the issue text.