[feat] add CI performance regression test for graph creation

[yuvraajnarula]

Describe your changes

This PR adds an automated performance regression test to the CI pipeline. It runs the graph creation benchmark on a fixed grid size (200×200) and compares runtime and peak memory against predefined thresholds. The test is triggered on every pull request that modifies src/ or tests/benchmarks/.

Changes include:

• New GitHub Actions workflow: .github/workflows/performance.yml
• New helper script: tests/benchmarks/check_regression.py to validate runtime and memory against thresholds
• The workflow uses the JSON output from graph_creation_scaling.py (added in [Feat] Memory Profiling and Automated Performance Regression Testing #137)

Thresholds (based on local runs with safety margin):

• Runtime: 25 seconds
• Peak memory: 500 MB

Why this is needed:

• Prevents performance regressions from being merged unnoticed.
• Provides an objective, automated check for graph creation efficiency.
• Complements the scaling benchmarks with a fixed‑point test suitable for CI.

Issue Link

Related to #137 (second part – automated regression testing)

Type of change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📖 Documentation

Checklist before requesting a review

• My branch is up-to-date with the target branch (main after [Feat] Memory Profiling and Automated Performance Regression Testing #137 is merged).
• I have performed a self-review of my code.
• For any new/modified functions/classes I have added docstrings (for check_regression.py).
• I have placed in-line comments where needed.
• I have updated the documentation (README.md – will add a note about the CI test).
• I have added tests that prove my fix is effective (the CI test itself is the test).
• I have given the PR a name that clearly describes the change, written in imperative form: "feat: add CI performance regression test for graph creation".
• I have requested a reviewer and an assignee.

Checklist for reviewers

• the code is readable
• the code is well tested
• the code is documented
• the code is easy to maintain

Author checklist after completed review

• I have added a line to the CHANGELOG describing this change, in the [Unreleased] section under added.

Checklist for assignee

• PR is up to date with the base branch
• the tests pass
• author has added an entry to the changelog
• Once the PR is ready to be merged, squash commits and merge the PR.