Add LinearSolver enum, remove boolean solver-selection flags

[bodono]

Summary

Replaces the boolean flags (gpu, mkl, apple_ldl, dense, cudss,
use_indirect) with a single linear_solver parameter that takes a
scs.LinearSolver enum value (also accepts strings).

Available solvers: AUTO, QDLDL, INDIRECT, MKL, ACCELERATE, DENSE,
GPU, CUDSS.

The default is AUTO, which auto-detects the best available direct solver:

• macOS: Apple Accelerate if built, otherwise QDLDL
• Linux/Windows: MKL Pardiso if built, otherwise QDLDL

Usage

import scs

# Auto-detect best backend (default)
solver = scs.SCS(data, cone)

# Explicitly select a solver
solver = scs.SCS(data, cone, linear_solver=scs.LinearSolver.MKL)

# String values also work
solver = scs.SCS(data, cone, linear_solver="indirect")

Implementation

Module selection uses dict dispatch for clean, extensible mapping:

_SOLVER_DISPATCH = {
    LinearSolver.AUTO: _resolve_auto,
    LinearSolver.QDLDL: lambda: _scs_direct,
    LinearSolver.INDIRECT: lambda: _load_module("_scs_indirect"),
    LinearSolver.MKL: lambda: _load_module("_scs_mkl"),
    ...
}

def _select_scs_module(stgs):
    linear_solver = stgs.pop("linear_solver", LinearSolver.AUTO)
    if isinstance(linear_solver, str):
        linear_solver = LinearSolver(linear_solver)
    return _SOLVER_DISPATCH[linear_solver]()

This is a breaking API change: the old boolean flags are removed, not deprecated.

Test plan

• All 250 tests pass with zero warnings
• Updated all 16 test files to use new linear_solver= API
• Tests for each LinearSolver enum value (QDLDL, INDIRECT, MKL, ACCELERATE, DENSE, GPU, CUDSS)
• Test string values work (linear_solver="qdldl")
• Test AUTO default works when no solver is specified
• Test invalid string raises ValueError

🤖 Generated with Claude Code