[FabianHofmann]

Note

The following content was generated by AI.

Stacked on #1052 (step 2b); base switches to master once #1052 is merged. Step 3 of #1019.

Closes #1045.

Changes proposed in this Pull Request

• Model.store_cls (DenseStore or CSRStore) is the single construction seam. Variable.to_linexpr, groupby(...).sum() and @/dot build on it through the store classmethods from_labels, from_grouper and the new from_matmul; the netcdf reader builds each linear expression through it, so a named expression of a sparse model reads back sparse unless it carries coordinates the sparse grid cannot hold, in which case it stays dense.
• Variable.to_linexpr loses its private _dense parameter. Quadratic products, rolling and cumsum build the dense expression explicitly through DenseStore.from_labels, without a notice.
• Store.to_sparse(reason) returns CSRStore | Reason: a dense store with coordinates the grid cannot hold refuses with coordinates the Grid cannot hold: <names> instead of dropping them. A mixed merge converts its dense operands through it and emits one Dense backing sparsified: merge with a sparse operand notice under warn_on_densify, only once the sparse merge succeeds; a refusal makes the whole merge dense with the named reason. A dense-only merge stays dense also in a sparse model. @ on a dense operand with such coordinates now keeps them on the dense path instead of dropping them.
• Dense groupby of a dense model stays on the xarray path: routing it through DenseStore.from_grouper changed dask laziness and aux-coordinate handling.
• Tests: conformance rows for mixed merges (sparse operand pulls sparse with one notice, dense operands around a sparse one, dense-only stays dense, dense operand with a 2-D coordinate falls back with the named reason, non-unique labels after conversion emit only the densify notice); to_linexpr returns the model's store class; netcdf round trip on both backends with and without a 2-D coordinate; matmul keeping a 2-D coordinate; Store.to_sparse refusal and notice.

Checklist

• AI-generated content is marked (see AGENTS.md).
• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[codspeed]

Merging this PR will not alter performance

✅ 181 untouched benchmarks
⏩ 181 skipped benchmarks¹

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_{Comparing store/3-construction-seam (c9c3bd3) with master (9a67427)}

Footnotes

1. 181 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩