docs: sparse_solver_roadmap.md attributes select_backend() to runner.py but it lives in gpu_backend.py

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Problem

docs/sparse_solver_roadmap.md (line 240) shows a code block titled:

Current auto-detection in runner.py

with a select_backend() function.

What the code shows

select_backend() is defined in vajax/analysis/gpu_backend.py (line 59), not in vajax/benchmarks/runner.py. It is exported via vajax/analysis/__init__.py (line 22 and 74).

vajax/benchmarks/runner.py contains no backend selection logic of this kind.

Impact

A developer trying to understand or modify the backend auto-selection logic will look in the wrong file.