Rmd-driven tinytest

Edit inst/tinytest/rmd/*.Rmd only. Generate test files with:

Rscript scripts/purl-tests.R

CI uses:

make check-purled-tests
make check-format
make check

Generated inst/tinytest/test-*.R files are committed to make tinytest::test_package() ordinary and transparent.

make check installs packages in dependency order, builds source tarballs in a temporary directory, and requires exactly Status: OK from every package check. Roxygen2 documentation is regenerated with make rd; the committed NAMESPACE and man/ files are generated artifacts.