Track benchmarks with Bencher #27

Reference: monoprop (.github/workflows/bench.yml, bench_main.yml, bench_bare_metal.yml).

We have benches/ and baseline-benchmarks/ but no continuous tracking.

• Register the project on Bencher, add BENCHER_API_TOKEN as a repo secret.
• Reusable bench.yml that runs the benchmarks and reports via BMF.
• bench_main.yml on pushes to main to build the historical baseline.
• PR workflow comparing against the baseline (alerts recorded, non-blocking at first).