…#816)

* perf: drop explicit zeros from the constraint matrix (#814)

Expressions that broadcast against a dense coordinate store one
coefficient per pair, most of them structurally zero. Those explicit
zeros were carried all the way into `matrices.A` and thus into every
solver handoff. `highspy.Highs.addRows` (and the other direct backends'
matrix loaders) scale with *stored* nnz, so the handoff spent most of
its work describing zeros — e.g. sparse_network(250) stored 1.5M entries
for 18k structural nonzeros (98.8% zeros).

Prune them once, centrally, in `_stack` via `eliminate_zeros()`, so `A`
and `indicator_A` — and hence HiGHS, gurobi, xpress, copt, mosek and the
LP/MPS writers — all hand the solver only structural nonzeros. A zero
coefficient never changes a constraint, so this is mathematically
identical.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* perf: prune explicit zeros at matrix assembly to cut build peak memory

Follow-up to #815. That eliminates zeros from the stacked constraint matrix
after scipy.vstack has already materialised the full dense-with-zeros block,
so the peak allocation (what the CodSpeed memory instrument tracks) is
unchanged — only the resting size and solver-ingest cost drop.

Prune at the source instead: fold `coeffs != 0` into the valid-entry mask in
Constraint._matrix_export_data, so broadcast zeros never enter cols/data/the
CSR. Snapshot capture and the matrix build share this path, so they stay
consistent.

Two ripples handled:
- matrices.dual derived active rows from stored nnz (np.diff(indptr)); an
  all-zero-coefficient row would lose its dual slot once pruned. Derive active
  rows from row activity via a new ConstraintBase.active_row_mask (also avoids
  rebuilding the CSR just to count rows).
- A zero-coefficient term no longer changes the sparsity pattern, so the
  persistent warm-start path no longer forces a SPARSITY rebuild for it. Test
  updated to use a non-zero coefficient; a no-rebuild case added.

sparse_network(250): build peak 50 -> 36 MB (~27%), identical solver result.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* ci: trigger CI

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Fabian Hofmann <fab.hof@gmx.de>