`filewriter`: append-only stream placeholder

Some datasets in ISIS NeXus files essentially contain the last known value of ie. a PV

the filewriter needs to be able to do this with a stream placeholder but instead of writing to a time-series array, just overwrite the value instead.

Locate the filewriter entry point and the existing stream-placeholder handling, then compare it with the time-series array path. Trace how values are written to ISIS NeXus files and confirm the expected behavior for last-known-value datasets. Done means a stream placeholder overwrites the current value instead of appending to a time-series array.

Written by the indexing model from the issue text.