Tracking: sparse/CSR path follow-ups after #961

This is a tracking issue for a series of complex refactors to the sparse/CSR data path in linopy. Start by reading the linked pull request #961 and the umbrella issue #756 to understand the context. The work is broken into three PRs, each with multiple parts (A-H) that touch core files like linopy/expressions.py, constraints.py, and test/test_csr.py. The first step is implementing observability (part A) to add shape/sizes/coords access and a warn_on_densify helper. 'Done' means all child issues are addressed and the sparse path preserves backing through operations without silent densification.

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Describe the feature you'd like to see

Tracking issue for the CSR follow-ups after #961 (umbrella #756 is closed). Each child names a place where a CSR-backed expression silently densifies, or a gap in the frozen-constraint API. This issue fixes the order in which to address them.

Key observations

• LinearExpression.data (linopy/expressions.py:2358) is where a CSR-backed expression densifies on read. Other paths drop the backing without passing through it: @ returns dense when the input was dense and sparse_groupby is off (expressions.py:2609), CSRConstraint.to_dense()/mutable(), the rebuild at model.py:1345, and the return None fallbacks in _sparse_matmul, _try_csr_merge, _aligned and csr_rhs, which lose the reason.
• Grid holds only per-dim indexes. Aux coords live next to it in CSRLinearExpression.coords and are dropped by CSRConstraint.from_csr (constraints.py:1401), which is #941.
• Zero policy on the sparse path: every operation keeps explicit zeros (merge, scaling, sum, groupby, selection), so sparse and dense give the same terms up to order; only @/dot prunes them. Cell activeness is carried by const alone. Export culls zeros regardless; #925 is an export/persistent-diff question and does not block this series.

Plan

Three pull requests, grouped by shared design rather than one per issue:

1. Expression kernel (parts A, C, B, D): observability, aux coords in Grid, and all operations that keep the backing. One commit per issue, so it can be reviewed commit by commit.
2. Boundaries (parts E, F): constraint side and export, after #806 has merged.
3. Control + docs (parts G, H): public-API decisions and documentation, once the preserving set is final.

Ordering notes:

• Within PR 1, commit in the order A, C, B, D. PR 1 stays out of constraints.py so #806 can land before PR 2.
• Tests extend the existing helpers in test/test_csr.py (assert_frozen_equal, assert_terms_equal, assert_cells_equal, assert_contracted_equal) with a "backing kept" assertion instead of adding a new fixture.
• Out of scope for now: DataArray sign in to_constraint and non-scalar fillna still densify. #745 is likely covered by sparse groupby; re-check and close or narrow.

Children

• #962
• #969
• #965
• #964
• #966
• #970
• #941
• #963
• #968
• #967
• #971