Add CodSpeed performance benchmarking

[codspeed-hq]

Summary

This PR sets up continuous performance measurement with CodSpeed for dataclassish.

It adds a benchmark suite covering the public multiple-dispatch API and wires up a GitHub Actions workflow so performance is measured automatically on every push to main and on pull requests.

Changes

• benchmarks/test_benchmarks.py: A pytest-codspeed benchmark suite exercising the public API across the object types dataclassish supports:
  • Mapping dispatches: replace, asdict, field_items
  • Dataclass dispatches: replace, fields, asdict, astuple, get_field, field_keys, field_values, field_items
  • Converter-enabled dataclass construction (converters.dataclass / field with Optional and Unless)
• .github/workflows/codspeed.yml: A CodSpeed workflow running in simulation mode. It authenticates via OpenID Connect (id-token: write), installs dependencies with uv, and runs the benchmarks with CodSpeedHQ/action. Actions are pinned by commit hash with version comments to match the existing workflow conventions.
• pyproject.toml / uv.lock: Added pytest-codspeed to the test dependency group.
• README.md: Added the CodSpeed badge.

Notes

• The benchmarks live in a dedicated benchmarks/ directory so they are not picked up by the regular pytest test run (testpaths remains unchanged). The workflow invokes them explicitly with -p no:sybil to avoid collecting doctest/README fixtures.
• The suite was validated locally with codspeed run --mode instrumentation and all 12 benchmarks were collected, executed, and uploaded successfully.

Next steps

• Merge this PR so CodSpeed records a baseline on main.
• Future PRs will automatically get a performance report comparing against that baseline.

[codspeed-hq]

Congrats! CodSpeed is installed 🎉

🆕 12 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_asdict_dataclass: 164.5 µs
• test_asdict_mapping: 185 µs
• test_astuple_dataclass: 141.8 µs
• test_converter_dataclass_construction: 93 µs
• test_field_items_dataclass: 204.9 µs
• test_field_items_mapping: 447.4 µs
• test_field_keys_dataclass: 190.5 µs
• test_field_values_dataclass: 193.7 µs
• test_fields_dataclass: 132.9 µs
• test_get_field_dataclass: 480.3 µs
• test_replace_dataclass: 131.2 µs
• test_replace_mapping: 192.2 µs

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