`SessionManager.close()` queues `#closePersistWriterInternal()` on the
persist chain. The task awaits `#persistWriter.close()`, which flips
`#closing = true` synchronously before yielding on its inner writer
`close()`. A concurrent `appendMessage()` landing in that yield window
hit the hot path, got the still-cached (but closing) writer back from
`#ensurePersistWriter()`, and threw `Error("Writer closed")` from
`writeSync`. The throw was stashed into `#persistError`; the next async
caller (`flush()` or a later `appendMessage()`) re-threw it as an
unhandled rejection with the original line-1282 stack.
Expose `NdjsonFileWriter.isOpen()` and treat a mid-close cached writer
as a miss in `#ensurePersistWriter()`. `_persist` now falls back to the
async `#rewriteFile()` cold path so the entry — already in
`#fileEntries` — still lands on disk once the close drains.
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