Make zero-length in-memory writes leave file size unchanged

An empty positioned write beyond EOF currently grows an InMem file and zero-fills the gap. Confirm the intended compatibility behavior with the original fs author, then make zero-length writes no-ops and add focused sparse-write coverage if appropriate. Deferred from #1318 to avoid changing observable fs semantics in the refactor.

First confirm the intended compatibility behavior with the original fs author, then inspect the InMem positioned-write path. Add focused sparse-write coverage if appropriate; done means a zero-length write beyond EOF leaves the file size unchanged and does not zero-fill the gap.

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