Add pull-request and main-branch continuous integration #8

Outcome

Every pull request and push to main receives the same fast correctness checks before release packaging.

Acceptance criteria

• A GitHub Actions workflow runs on pull requests and pushes to main.
• It uses a clean dependency install and supported Node version.
• It runs syntax/type checks, the full automated test suite, and whitespace/diff validation.
• Concurrency cancels superseded runs where appropriate.
• The release-tag workflow remains responsible for packaging and publication.
• Required-check and branch-protection recommendations are documented.