Add OpenBLAS-vs-JIT crossover benchmark

[MesTTo]

Depends on #111 (OpenBLAS backend) and #115 (einsum-f32 JIT); both appear in the diff until they merge, after which this is just the crossover benchmark.

Adds a benchmark that sweeps matrix density and compares the OpenBLAS dense backend against the einsum-f32 JIT, so the crossover point (where sparse JIT overtakes dense BLAS) is measurable. Behind jit + blas.

Validation:

• CARGO_HOME=/home/user/Dev/.cargo-isolated RUSTFLAGS='-C target-cpu=native -Awarnings' cargo +nightly build -p linalg --features "jit blas" --benches
• git diff --check

[MesTTo]

this one genuinely needs both backends to compare them (openblas from #111 and the jit from #115), so it can't apply independently and bundling all three into one diff isn't reviewable. holding it as a small standalone bench to add the moment #111 and #115 are both in, rather than stacking. closing for now.