Widen the CSRConstraint fast path in Model.add_constraints

[FabianHofmann]

Note

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Describe the feature you'd like to see

Widen the conditions under which Model.add_constraints keeps a CSRConstraint. Child of #756.

The sparse constraint path is narrow. add_constraints keeps the sparse object only when isinstance(con, CSRConstraint) and freeze and mask is None (linopy/model.py:1333). Any mask=, any penalty=, freeze=False (the default), or a set self.chunk reconstructs the full dense rectangle at model.py:1345.

Upstream of that, csr_rhs (linopy/constraints.py:1427-1441) returns None for a non-constant right-hand side, or one carrying helper dims or dims outside the grid, so to_constraint falls back to dense. An rhs that is itself an expression is a common case and always densifies.

The result is that a build which stayed sparse through every expression operation can still densify at the last step, silently.

Implementation ideas

• mask=: a mask is a row selection on the CSR store, the same primitive the sparse where needs. These two should probably be implemented together.
• Expression rhs: move the rhs to the left-hand side before building the constraint, which is what the dense path effectively does, then the existing constant-rhs path applies.
• penalty/soft constraints are deliberately incompatible with freezing today (model.py:1313); worth confirming that is a permanent rule rather than a gap.

Separately, note that the sparse route currently needs three independent opt-ins to work end to end: semantics="v1", then sparse_groupby=True or a per-call sparse=True, then freeze_constraints=True. Reducing that to one decision would be worth discussing under #756.