perf: direct CSR-to-LP writer for frozen constraints

[FBumann]

Summary

• Override Constraint.to_polars() to expand CSR data directly into a polars DataFrame, bypassing the expensive mutable() → xarray Dataset reconstruction
• Override Constraint.iterate_slices() to yield CSR row-batches instead of relying on xarray's isel() (which doesn't exist on frozen constraints — this was crashing)
• Move eliminate_zeros() to freeze time (from_mutable) so cleanup happens once

Benchmark results (vs master)

Benchmark	master	PR #630 (before)	+ this PR	vs master
basic n=100	12.5ms	14.1ms (+12%)	8.5ms	-32%
expr_arith n=100	16.5ms	14.7ms (-11%)	10.7ms	-35%
knapsack n=10k	4.2ms	4.8ms (+16%)	3.3ms	-21%
sparse_net n=100	5.4ms	7.9ms (+47%)	4.6ms	-15%
pypsa_scigrid 10	—	crashed	32.0ms	fixed

Test plan

• pytest test/test_io.py — all 25 tests pass
• pytest test/test_constraint.py — all 72 tests pass
• New test test_to_file_lp_frozen_vs_mutable verifies byte-identical LP output between frozen and mutable paths
• benchmarks/test_lp_write.py — all benchmarks pass including previously crashing pypsa_scigrid

🤖 Generated with Claude Code

[coroa]

thank you

[FabianHofmann]

this is already nice! I made some LP writing experiment in https://github.com/FabianHofmann/crs2lp which showed quite some improvements in performance. It directly writes the LP file from the CRS objects using custom rust module.

That said, I am happy to pull this one in to keep changes small but in the long term it is probably worth to move the implementation in the repo.

[FabianHofmann]

@coroa this is great, one big question that came up when looking at the code is: why renaming Constraint to MutableConstraint and making Constraint the frozen one, and not instead keep Constraint and add a FrozenConstraint? From a user perspective the current renaming is breaking and the exposed object has a non-intuitive name.

[FabianHofmann]

@coroa this is great, one big question that came up when looking at the code is: why renaming Constraint to MutableConstraint and making Constraint the frozen one, and not instead keep Constraint and add a FrozenConstraint? From a user perspective the current renaming is breaking and the exposed object has a non-intuitive name.

this should be in the other pr, moving it there (sorry)