ci: add PR coverage check workflow

[vividctrlalt]

Summary

• Add a new GitHub Actions workflow (coverage-check.yml) that compares code coverage between PR branch and base branch
• Runs JaCoCo test coverage on both branches in parallel, then compares results
• Posts a coverage report as a PR comment with per-module breakdown
• Fails the check if line coverage drops more than the configurable threshold (default: 1%)

Test plan

• Open a test PR against develop to verify the workflow triggers
• Verify JaCoCo reports are generated and uploaded as artifacts
• Verify the coverage comparison comment is posted on the PR
• Verify the check fails when coverage drops beyond threshold