Update LIPIcs template to v2021.1.3

[cderv]

Summary

• update the bundled LIPIcs author resources to the authoritative Dagstuhl v2021.1.3 tag (lipics-v2021.cls v3.1.3)
• add v2021 metadata and theorem/proof interfaces while preserving Pandoc compatibility blocks, include hooks, bookdown behavior, appendix placement, and the LaTeX 2026 thm-restate workaround
• keep citation_package user-selectable and document the official BibTeX/plainurl workflow versus Pandoc citation processing
• switch the default engine to pdfLaTeX after testing both pdfLaTeX and XeLaTeX; XeLaTeX remains selectable
• preserve old drafts through a v2019 class fallback, including normalization of their legacy Unicode thin-space metadata and compatible rendering of v2021-only metadata

The publisher v2021 class, changelog, and three runtime PDF assets match the tagged archive byte-for-byte. The LPPL file and sample bibliography were text-identical after line-ending normalization and were left unchanged. Generated sample and author-guideline PDFs are not bundled. The previous v2019 class is retained outside the new skeleton as a legacy regression fixture and keeps its LPPL notice.

Citation and engine findings

Raw LaTeX \cite{...} citations with the extensionless bibliography value follow the official BibTeX route and retain \bibliographystyle{plainurl}. Markdown citations under citation_package = "natbib" become natbib commands, but this custom template does not load natbib. citation_package = "default" uses citeproc and requires the .bib extension in this skeleton; keys used only by structured related-version or supplement cite fields must also be listed under YAML nocite, as documented and tested.

pdfLaTeX passed the normal skeleton, bibliography workflow, structured v2021 metadata, oldauthorstyle, PDF/A, and theorem/proof features. XeLaTeX passed the normal updated skeleton, but the tagged class's PDF/A path fails at \pdfobjcompresslevel; this limitation is documented.

Tests

• full testthat suite
• full testit format-render suite with NOT_CRAN=true
• focused LIPIcs renders: default pdfLaTeX, XeLaTeX, v2021/PDF-A features, citeproc with structured-key nocite, legacy v2019 class with old   metadata, and thm-restate
• Pandoc 3.1.7 citeproc render for the standard-list replacement of the discouraged enumitem block
• two R CMD check --no-manual runs: 0 errors, 0 warnings, 1 pre-existing .private-journal note

Dedicated pre-2026/current-2026 TeX snapshot CI was investigated using supported monthly TinyTeX pins, but is intentionally not added because rticles does not maintain per-format TeX-version matrices. Existing CI and the focused thm-restate render continue to exercise the current TeX environment.

Closes #608

[cderv]

Addressed the three follow-up review findings in the force-updated commit:

• Legacy pdfLaTeX drafts: the template now maps U+2009 (produced by the old   metadata) to \, for pdfLaTeX. A render test uses the exact retained v2019 class and old author-running metadata.
• v2019 fallback: class selection is tracked explicitly. Subtitle is folded into the legacy title, and structured related-version/supplement records degrade to the supported v2019 macros rather than invoking undefined v2021 commands.
• Structured citeproc keys: the skeleton and wrapper docs now require keys used only by structured cite fields to be listed under YAML nocite; the citeproc render validator confirms the structured key appears in the generated CSL references.

The post-fix full testthat suite, focused renders, Pandoc 3.1.7 citeproc render, and R CMD check all pass. The check result remains 0 errors, 0 warnings, and the pre-existing .private-journal note.