Add CI/CD pipeline with R CMD check and code coverage

[jasdumas]

Problem

No automated testing or code coverage reporting. PRs and pushes should be validated automatically.

Proposed Solution

• Add GitHub Actions workflows:
  • R CMD check on ubuntu, macOS, windows (R-release, R-devel, R-oldrel)
  • Code coverage via covr → Codecov
  • pkgdown site build (already created, #in-progress)
  • Lint via lintr for style consistency
• Add badges to README: R CMD check, coverage, CRAN status, lifecycle

Workflow Files

• .github/workflows/R-CMD-check.yaml — multi-platform check matrix
• .github/workflows/test-coverage.yaml — covr + Codecov
• .github/workflows/lint.yaml — lintr

Acceptance Criteria

• R CMD check passes on 3 platforms × 2 R versions
• Code coverage > 80% and reported on Codecov
• README badges reflect current status