Add CodSpeed performance benchmarks and CI workflow

[codspeed-hq]

Summary

This PR integrates CodSpeed for continuous performance tracking of hpke-http's cryptographic operations.

What's included

• 13 benchmarks covering the performance-critical paths:
  • HPKE sender/recipient context setup (key exchange + key schedule)
  • Single-shot seal_psk / open_psk with small JSON and 4 KB payloads
  • Context-based seal/open (amortised key exchange cost)
  • Chunk encryption/decryption in both SSE and raw binary formats (64 KB)
  • Gzip compression/decompression at 4 KB and 64 KB sizes
• GitHub Actions workflow (.github/workflows/codspeed.yml) running benchmarks in simulation mode on push to main/develop and on PRs to main
• CodSpeed badge added to the README
• pytest-codspeed added as a dev dependency

How it works

CodSpeed's simulation mode uses CPU instruction counting for deterministic, low-variance measurements. Every PR will get a performance report comparing against the base branch, catching regressions before they land.

Next steps

• Merge this PR and run the CodSpeed workflow on main to establish baseline measurements
• Future PRs will automatically receive performance comparisons in the PR checks
• Visit the CodSpeed dashboard to explore results and flamegraphs

[codspeed-hq]

Congrats! CodSpeed is installed 🎉

🆕 13 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_gzip_compress_64k: 4.3 ms
• test_bench_gzip_compress_4k: 338.9 µs
• test_bench_chunk_encrypt_sse: 1.1 ms
• test_bench_chunk_encrypt_raw: 435.1 µs
• test_bench_context_open: 1.3 ms
• test_bench_setup_recipient_context: 603.9 µs
• test_bench_seal_psk_small: 814 µs
• test_bench_context_seal: 807.9 µs
• test_bench_chunk_decrypt_raw: 733.1 µs
• test_bench_seal_psk_medium: 823 µs
• test_bench_setup_sender_context: 795.4 µs
• test_bench_gzip_decompress_4k: 86.2 µs
• test_bench_open_psk: 633.2 µs

---