Keep CSR backing through where/mask/sel/isel/loc

[FabianHofmann]

Note

The following content was generated by AI.

Describe the feature you'd like to see

Keep the CSR backing through selection and masking: where, mask, sel, isel, loc and __getitem__. Child of #756.

All of them densify today: where (linopy/expressions.py:1892-1942), mask (1666), sel/isel and the rest of the exprwrap bindings (2245-2297), __getitem__ (1537-1544). Conditional constraints appear in nearly every real model, so this is on the common path.

On a CSR store these are among the cheapest operations available. Masking is a row selection plus a const write, since absence is already carried by a NaN const. Label selection is a reindexed onto a narrower grid, which already exists. Positional selection is a row gather.

Implementation ideas

• where(cond): evaluate cond over the grid, then write NaN into const for the false cells. The v1 rule that an absent cell has no terms is already implemented for added (linopy/csr.py:395-398), so the term arrays may be left untouched or pruned, whichever matches that invariant.
• sel/loc: build the target Grid and route through the existing reindexed.
• isel/__getitem__: a row gather on indptr plus a new Grid from the selected indexes.

Note the ordering interaction with #925: pruning versus preserving explicit zeros should follow whatever policy that issue settles.