feat(format): read the other attributes when one cannot be read

attrs() read every attribute of an object through extract_attributes_full,
so one attribute it could not read (a corrupt or unsupported attribute
message, or a heap object it could not locate) failed all of them — the
same shape as the huge-object bug, where one 8 KiB attribute hid every
attribute on a NetCDF file's root group.

- clawhdf5-format: new attribute::extract_attributes_tolerant returns the
  attributes it could read plus one error per attribute it could not.
  Errors in the attribute index itself (Attribute Info message, dense
  heap header, B-tree) still fail, since then it is unknown which
  attributes exist. extract_attributes_full is unchanged (strict); both
  share one implementation.
- clawhdf5: attrs() on Group/Dataset, MmapGroup/MmapDataset and
  LazyGroup/LazyDataset leaves an unreadable attribute out (documented),
  and the new attrs_with_errors() returns the map with the per-attribute
  errors. A value is either returned complete or not at all.

Regression test: one_unreadable_attribute_does_not_hide_the_others (h5py
writes 11 dense attributes; one message's version byte is corrupted;
before: attrs() failed with InvalidAttributeVersion(127), after: the 10
others come back with their values and one error is reported).

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

CHANGELOG.md

@@ -337,6 +337,14 @@
   old-style (symbol table) groups a soft link made the listing fail. New
   `group_v2::resolve_group_children` / `resolve_path_from` and
   `group_v1::v1_soft_links` in `clawhdf5-format`.
+- `clawhdf5` — one unreadable attribute no longer fails `attrs()` for every
+  attribute on its object: it is left out of the map, and the new
+  `attrs_with_errors()` (on every group and dataset handle) returns the map
+  plus one error per attribute left out. Returned values are always complete.
+  An error in the attribute index itself (attribute info message, dense heap
+  header or B-tree) still fails the call. `clawhdf5-format` gains
+  `attribute::extract_attributes_tolerant`; `extract_attributes_full` stays
+  strict.
 - `clawhdf5-format` writer — **files libhdf5 rejects or reads wrong:**
   - Extensible Array (one unlimited dimension): chunks from index 244 on were
     written but never indexed and read as 0, by libhdf5 and by us.