Add tests and CI; move data into data/ and images/

[macromeer]

Summary

Follow-up to #2, which was merged before these two commits were pushed.

Tests and CI

• A testthat suite in tests/testthat/ that checks:
  • Data: every CSV value is read, including the headerless data_C files.
  • Panels: each one renders without warnings, plus targeted checks for the bugs fixed in Fix plotting bugs, update for ggplot2 4.x, save figure from script #2: Panel A labels, Panel B edge bars and single curve, Panel C point count and fixed jitter, Panel D overlay bounds, Panel E error bars.
  • Export: PNG pixel size, the SVG font fallback list, and no Type 3 fonts in the PDF.
  • Visual snapshots: vdiffr snapshots of panels B–F. Panel A is left out so the microscopy images aren't copied into the snapshots.
• figure_example.R has two options for testing: options(scifig.save = FALSE) builds the panels without writing files, and options(scifig.out_dir = ...) writes the files to another folder.
• DESCRIPTION lists the dependencies. This is not an R package; the file lets CI and pak::local_install_deps() install them.
• A GitHub Actions workflow (.github/workflows/tests.yaml) runs the tests on every push to main and every pull request.

Layout

• The CSVs move to data/ and the microscopy images to images/. Git records these as renames, so file history is kept.
• Paths are updated in the script and tests, and the README describes the layout and how to run the tests.

Test plan

• Rscript -e 'testthat::test_dir("tests/testthat")': 48 passed, 0 warnings
• Re-introducing four of the fixed bugs (CSV header, Panel D bounds, Panel B edge bars, Panel A labels) makes the tests fail
• Rscript figure_example.R writes identical PNG and SVG after the file move
• CI passes on this PR (48 passed, 0 skipped)

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