Setup CodSpeed for continuous performance monitoring

[codspeed-hq]

Summary

• Replaced criterion = "0.3" with codspeed-criterion-compat = "4.3.0" using Cargo dependency renaming. This means the benchmark code (bench/benches/bench.rs) requires zero changes -- it continues using use criterion::*; as before.
• Added a new GitHub Actions workflow (.github/workflows/codspeed.yml) that builds and runs the benchmarks with CodSpeed on every push to main, on pull requests, and on manual dispatch. Uses OIDC authentication.
• Added CodSpeed badge to the README.

Changes

• bench/Cargo.toml: Updated criterion dependency to use codspeed-criterion-compat v4.3.0 via package renaming
• Cargo.lock: Updated lockfile to reflect new dependency tree
• .github/workflows/codspeed.yml: New workflow for CodSpeed benchmarks (simulation mode, OIDC auth, ubuntu-latest runner)
• README.md: Added CodSpeed badge

Next steps

• Merge this PR to start collecting baseline performance data on the main branch
• CodSpeed will automatically report performance changes on future PRs
• Visit CodSpeed dashboard to view benchmark results after the first run

[codspeed-hq]

Congrats! CodSpeed is installed 🎉

🆕 29 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

• num-prime (this crate): 24.7 ms
• glass_pumpkin: 6.7 µs
• glass_pumpkin: 265.1 µs
• glass_pumpkin (BPSW): 6.3 µs
• num-prime (this crate): 119.9 ms
• number-theory: 39.5 ms
• num-primes: 34.4 µs
• glass_pumpkin: 214.9 ms
• num-prime (this crate): 2.4 ms
• num-primes: 1.9 s
• num-prime (this crate): 116.9 ms
• num-prime (this crate): 2.5 ms
• glass_pumpkin: 11.7 µs
• num-primes: 2 ms
• num-primes: 3 ms
• glass_pumpkin (BPSW): 6.7 µs
• num-primes: 184.9 µs
• glass_pumpkin: 3.7 ms
• glass_pumpkin: 30.9 ms
• num-primes: 23 ms
• ...

ℹ️ Only the first 20 benchmarks are displayed. Go to the app to view all benchmarks.

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