clawhdf5: FileEditor reports filters it cannot run as Error::Unsupported

A dataset whose filter this build cannot encode (scale-offset, N-Bit, SZIP;
a plugin filter the build lacks) failed with Error::Format("unsupported
filter: 6"), although the editor documents every refused edit as
Error::Unsupported, and the Python bindings raised ValueError rather than
NotImplementedError. Every edit now maps FormatError::UnsupportedFilter to
Error::Unsupported; the file is left untouched as before.

Test: edit_interop unencodable_filters_are_unsupported — h5py scale-offset
datasets (integer with chunks, integer never written, float D-scale):
Error::Unsupported naming the filter, and the file byte for byte unchanged.
Fails on the previous editor (Format(UnsupportedFilter(6))).

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

docs/known-issues.md

@@ -13,11 +13,16 @@ deleting it.
 with `Error::Unsupported` and without writing anything:
 - new, moved or resized chunks in a **version-2 B-tree** chunk index (what
   libhdf5 uses for two or more unlimited dimensions) — existing unfiltered
-  chunks, and filtered ones that re-encode to the same size, are
+  chunks, and filtered ones that re-encode to the same size and filter
+  mask, are
   overwritten in place; `resize` works — and new chunks in an **implicit**
   index (it has all of its chunks from the start);
 - **shrinking** a dataset;
 - variable-length and reference data;
+- chunks through a filter this build cannot encode (scale-offset, N-Bit,
+  SZIP, or a plugin filter it lacks), even an optional one: libhdf5 skips
+  an optional filter only when its own build lacks it, which none does for
+  these;
 - attributes of an object in **dense storage**, past its compact limit (8
   by default) or with tracked **creation order**;
 - partial edge chunks stored unfiltered (`H5Pset_chunk_opts`), external