ci: add automated performance benchmark workflow for pull requests

[AKonnyaku]

This PR introduces a new CI workflow to monitor performance.

• Workflow integration: the system triggers a benchmark test for every new pull request. It runs on both the target branch and the current branch.
• Performance comparison: this setup compares the execution time between versions. It helps developers identify potential performance regressions immediately.
• Implementation detail: a python script parses the json output from pytest. It calculates the geometric mean to produce a readable comparison table.

Fixes #417