Add CodSpeed performance benchmarks

[codspeed-hq]

Summary

This PR integrates CodSpeed for continuous performance testing of the OSQP Python solver. It adds a benchmark suite covering the core solver operations and a GitHub Actions workflow that runs these benchmarks on every push to master and on pull requests.

Changes

Benchmark suite (benchmarks/bench_solver.py)

11 benchmarks using pytest-codspeed (compatible with pytest-benchmark API) covering:

• Setup: Solver initialization across small (n=2), medium (n=50), and large (n=200) QP problems
• Solve: Solving QP problems at each size
• Update + solve: Updating linear cost and bounds, then re-solving
• End-to-end: Full setup and solve cycle (small and medium)
• Polishing: Solve with the polishing refinement step enabled
• Warm start: Solve with warm starting from a previous solution

CI workflow (.github/workflows/codspeed.yml)

• Runs on push to master, pull requests, and workflow_dispatch (for CodSpeed backtesting)
• Uses OIDC authentication (no token needed)
• Builds the C++ extension from source, then runs benchmarks in simulation mode
• Uses CodSpeedHQ/action@v4

README

• Added CodSpeed badge

Next steps

Once this PR is merged:

1. Performance results will be tracked on the CodSpeed dashboard
2. Pull requests will receive automated performance reports comparing against the base branch
3. Performance regressions will be flagged before they reach master

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[CLAassistant]

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[codspeed-hq]

Congrats! CodSpeed is installed 🎉

🆕 11 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• test_bench_end_to_end_medium: 4.8 ms
• test_bench_solve_medium: 808.4 µs
• test_bench_end_to_end_small: 2 ms
• test_bench_solve_small: 159.4 µs
• test_bench_setup_large: 18.9 ms
• test_bench_solve_with_polishing: 1.3 ms
• test_bench_setup_medium: 3.5 ms
• test_bench_update_vectors: 888.3 µs
• test_bench_setup_small: 2 ms
• test_bench_warm_start_solve: 825.8 µs
• test_bench_solve_large: 6.4 ms

---

[codspeed-hq]

Congrats! CodSpeed is installed 🎉

🆕 11 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• test_bench_end_to_end_small: 2 ms
• test_bench_setup_small: 2 ms
• test_bench_setup_large: 19.1 ms
• test_bench_solve_large: 6.4 ms
• test_bench_setup_medium: 3.5 ms
• test_bench_solve_medium: 808.7 µs
• test_bench_warm_start_solve: 826.2 µs
• test_bench_solve_small: 159.9 µs
• test_bench_solve_with_polishing: 1.3 ms
• test_bench_update_vectors: 886.9 µs
• test_bench_end_to_end_medium: 4.8 ms

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[vineetbansal]

Note: This PR is experimental, to test out a benchmarking framework I recently learned of. DO NOT MERGE!