Make expandable a list; default expands only DynamicTable columns

[bendichter]

Summary

• Change the default expandable behavior in HDF5IO.write / write_builder / write_group / write_dataset so that only DynamicTable columns and id are made expandable out of the box. Datasets of all other types now default to fixed-shape on-disk layout.
• Breaking change: expandable is now a list (or tuple) of data type names with default ("VectorData", "ElementIdentifiers"), not a boolean. A dataset is made expandable if its data type (or an ancestor) is in the list — so subclasses of VectorData (e.g. DynamicTableRegion) and ElementIdentifiers are also covered. Pass an empty list/tuple to disable automatic expansion entirely. Passing True/False now raises a TypeError from docval.
• Subclass detection uses BuildManager.is_sub_data_type, with a get_builder_dt short-circuit so untyped dataset builders don't hit the hierarchy lookup.
• CHANGELOG entry added under a new Breaking changes heading with migration guidance.

Closes #1437

Test plan

• TestDefaultExpandable — VectorData, VectorIndex, indexed VectorData, and ElementIdentifiers are expandable by default
• TestDefaultExpandableSubclasses — user-defined subclasses of VectorData / ElementIdentifiers are expandable by default (regression test for the is_sub_data_type fix)
• TestDefaultExpandableWithReferences — reference-column dataset carries a reference dtype, is expandable, and refs resolve round-trip
• TestDefaultExpandableExplicitOverride — explicit H5DataIO(maxshape=...) is preserved, not overridden
• TestExpandableArg — expandable=[] disables expansion; custom list expands only listed types; expandable=True/False raises TypeError
• TestExpand.test_expand_set_shape — expandable=['QuxData'] still supports the append path on non-table types
• Existing TestDTRReferences — DynamicTableRegion with references still works
• Full test suite passes locally

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.21%. Comparing base (15f1d77) to head (31e453c).

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1439      +/-   ##
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+ Coverage   93.19%   93.21%   +0.01%     
==========================================
  Files          41       41              
  Lines       10062    10062              
  Branches     2069     2069              
==========================================
+ Hits         9377     9379       +2     
  Misses        409      409              
+ Partials      276      274       -2     

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[rly]

Looks good. I made some small changes:

Summary

• Fixed ElementIdentifiers subclass detection in HDF5IO.write_dataset: replaced name equality (builder_dt == 'ElementIdentifiers') with self.manager.is_sub_data_type(builder, 'ElementIdentifiers') so subclasses are also made expandable by default. Updated the matching docstring wording in the four expandable docval blocks.
• Added CHANGELOG.md entry under Unreleased → Fixed describing the full default-expandable behavior change.

Tests added

• TestDefaultExpandableSubclasses (in tests/unit/common/test_table.py) — regression test for the is_sub_data_type fix. Uses create_load_namespace_yaml to define CustomElementIdentifiers and CustomVectorData subclasses, writes a DynamicTable using them, and asserts both id and custom_col get maxshape=(None,).
• TestExpandableFalse — asserts expandable=False yields fixed shapes (2,) for both a VectorData column and the ElementIdentifiers id.
• test_docval_none_default_with_non_none_type (in tests/unit/utils_test/test_docval.py) — MWE confirming docval accepts default=None with type=bool (default preserved, wrong types still rejected), justifying the narrow 'type': bool in the expandable docval.
• test_roundtrip_basic_append — now passes expandable=True explicitly and asserts f['.../my_data'].maxshape == (None,) to cover the force-all path on a non-VectorData dataset.

[oruebel]

Looks good to me. I'll let @rly do final approval

[rly]

I updated the changelog and moved the tests to a more appropriate location since expandable is no longer specific to tables. I also updated the PR summary so that the squash commit reflects the current state of changes. This is technically a breaking change and #1440 will be a major change in behavior, so the next release will be 6.0.0.