py: boolean-mask keys raise NotImplementedError, not TypeError

h5py supports boolean masks for reads and writes; clawhdf5 supports
neither, so a mask is an unsupported operation (NotImplementedError, as
for every other edit the bindings cannot do), not an invalid key.

Tests: test_unsupported_edits_are_clear_errors (1-D, N-D and per-axis
mask writes, file unchanged) and test_boolean_masks_are_refused (reads).

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

docs/known-issues.md

@@ -164,8 +164,9 @@ before anything is written. On top of them:
   elements, variable-length data, strings padded with spaces or
   NUL-terminated (libhdf5 converts those differently from numpy; NUL-padded
   ones, h5py's, are writable), compounds containing such strings, null
-  dataspaces, and index-list writes of more than 2²² elements (write them
-  in slices).
+  dataspaces, index-list writes of more than 2²² elements (write them
+  in slices), and boolean-mask keys (`ds[mask] = v`, and mask reads;
+  h5py supports both).
 - **`str` attributes are fixed-length UTF-8**, where h5py writes
   variable-length strings: h5py reads them back as `bytes`
   (`numpy.bytes_`), not `str`.