Add CodSpeed benchmark CI

[rudrakshkarpe]

Summary

• Add a CodSpeed GitHub Actions workflow for PR and main-branch benchmark runs
• Add pytest-codspeed as an optional codspeed extra
• Add deterministic core scoring/report rendering benchmarks and local README instructions

Closes #16

Validation

• .venv/bin/python -m pytest benchmarks/ --codspeed
• .venv/bin/python -m ruff check .
• .venv/bin/python -m pytest
• .venv/bin/python -m mypy src
• rm -rf dist && .venv/bin/python -m build && .venv/bin/python -m twine check dist/*

[rudrakshkarpe]

CodSpeed benchmark tests are executing successfully in GitHub Actions, but the upload is currently blocked by CodSpeed auth:

Failed to retrieve upload data: 401 Unauthorized
Reason: Repository not found or the user does not have access to it.

The workflow now uses the recommended OIDC permissions. The remaining setup step is to import/authorize rudrakshkarpe/dual-llm-bench in the CodSpeed dashboard/GitHub App, then re-run the failed CodSpeed job.