Make BLIS reproducers enforce actual backend selection and fix C runner portability - #34169

The reproducer workflow was not reliably demonstrating the intended BLAS scenario, and the Python/C reproducers had drifted from the requested setup. This update tightens both reproducers so runs fail when the requested backend is not actually in effect and removes path assumptions that broke the C job.

• C reproducer: path-safe and self-contained

  • Remove hardcoded checkout path usage in build_and_run_c_blis_reproducer.sh.
  • Resolve source paths relative to the script directory.
  • Add the missing blis_gemm_reproducer.c source used by the script.
  • Keep BLIS configure invocation in working order (./configure --enable-cblas auto).

• Python reproducer: backend verification, not just execution

  • Add --expected-blas to numpy_blis_reproducer.py.
  • Detect NumPy BLAS backend at runtime and fail on mismatch.
  • Print detected backend in output for fast triage.

• Runner behavior alignment

  • Update run_numpy_blas_reproducer.sh to pass --expected-blas explicitly.
  • Correct thread env handling so newaccelerate does not inherit OpenBLAS/BLIS thread vars.
  • Refresh README wording to reflect backend verification behavior.

# launcher now enforces backend intent
./build_tools/github/blis_reproducer/run_numpy_blas_reproducer.sh blis

# python reproducer now validates backend selection
python build_tools/github/blis_reproducer/numpy_blis_reproducer.py --expected-blas blis