Encode structure viewer representation styles

[cpignedoli]

Summary

This PR adds an extxyz-safe naming convention for structure-viewer representation arrays.

New representation arrays now encode their display style in the array name, for example:

_aiidalab_viewer_representation_spacefill_r3_element_<token>
_aiidalab_viewer_representation_ballstick_r4_red_<token>

The existing per-atom membership values remain unchanged (1 for included atoms, -1 for excluded atoms).

Backward compatibility

Old opaque representation names, such as _aiidalab_viewer_representation_nQ34..., are still accepted. If a name cannot be parsed, the viewer keeps the historical default style settings and does not rename it just by loading the structure.

Details

• Adds encode_representation_style_id() for generating safe, style-aware representation array names.
• Adds parse_representation_style_id() for restoring type, size, and color from encoded names.
• Keeps a uniqueness token so two atom groups can share the same visual style without colliding.
• Refreshes encoded array names on Apply when the representation type, size, or color has changed.
• Imports multiple representation arrays as a batch, so loading one representation no longer prunes other representation arrays that have not been imported yet.

This PR is stacked on #766, which fixes importing representation arrays that are present in loaded structures.

Representation PR chain

• PR-R1: this PR, encoded representation style IDs.
• PR-R2: PR-R2: Load viewer representations from extras #769, load representation masks from AiiDA extras and drop stale rows.
• PR-R3: PR-R3: Persist viewer representations in extras #770, persist representation masks back to AiiDA extras.
• Surfaces AutoRep integration: Use encoded style ids for AutoRep representations nanotech-empa/aiidalab-empa-surfaces#297

Validation

• python -m pytest tests/test_viewers.py::test_structure_data_viewer_representation tests/test_viewers.py::test_structure_data_viewer_imports_unknown_representation_array tests/test_viewers.py::test_structure_data_viewer_imports_encoded_representation_array tests/test_viewers.py::test_structure_data_viewer_imports_multiple_encoded_representation_arrays tests/test_viewers.py::test_structure_data_viewer_updates_encoded_representation_array_name
• python -m pytest tests/test_viewers.py

Implemented with Codex.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.40%. Comparing base (42ac05e) to head (0ea73c4).
⚠️ Report is 1 commits behind head on fix/structure-viewer-representation-style-id.

Additional details and impacted files

@@                               Coverage Diff                                @@
##           fix/structure-viewer-representation-style-id     #768      +/-   ##
================================================================================
+ Coverage                                         85.03%   85.40%   +0.37%     
================================================================================
  Files                                                18       18              
  Lines                                              3335     3379      +44     
================================================================================
+ Hits                                               2836     2886      +50     
+ Misses                                              499      493       -6     

Flag	Coverage Δ
python-3.12	85.38% <100.00%> (+0.43%)	⬆️
python-3.9	85.39% <100.00%> (+0.37%)	⬆️

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