Add CPU contraction benchmark harness

[GiggleLiu]

Summary

• Add a release-mode CPU contraction benchmark example covering RHS transpose views, batch-major batched contractions, and optimized-root output permutations.
• Wire make bench to the benchmark example with tunable make variables.
• Update the performance guide with benchmark usage and corrected optional parallel feature guidance.

Test Plan

• cargo fmt --all -- --check
• cargo check --example cpu_contract_bench
• cargo clippy --example cpu_contract_bench --features tropical,parallel -- -D warnings
• make bench BENCH_SCENARIO=all BENCH_ITERATIONS=1 BENCH_WARMUP=0 BENCH_DIM=4 BENCH_BATCH=2
• make check
• cargo build --release --example cpu_contract_bench

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[GiggleLiu]

Merged the PR 40 benchmark harness into PR 29 via commit 9a2a67a, so closing this duplicate follow-up PR.