Improve BibTeX parsing and bibliography rendering

Improve Scribble's support for rich bibliographies, particularly annotated bibliographies with long, formatted, multi-paragraph notes. The immediate motivation is the bibliography of my book Lambda: the Ultimate Object, but the improvements apply to ordinary academic papers as well.

BibTeX parsing

• Add a small LaTeX reader supporting common formatting commands, accents, special characters, protected groups and mathematical expressions.
• Preserve protected groups when parsing author names and other fields.
• Support LaTeX escapes in URLs and DOIs, generating correct hyperlinks in HTML and LaTeX.
• Improve parsing of BibTeX string references, including references adjacent to delimiters.
• Support formatted content and multiple paragraphs in bibliography notes.

The reader deliberately supports a useful subset of LaTeX rather than attempting to implement a complete TeX interpreter. Unknown commands are preserved rather than silently discarded.

Bibliography rendering

• Replace the old LaTeX \parbox layout, which prevented page breaks within bibliography entries, with needspace, allowing long annotations to span pages. Keep a five-line minimum by default to approximate the previous behavior.
• Make needspace optional and expose configurable page-breaking and line-breaking settings without imposing new line-breaking parameters on existing documents.
• Improve numbered bibliography layout using CSS and LaTeX environments, with hanging indentation and aligned citation numbers.
• Improve formatting across bibliography fields.
• Add a period after a DOI only when a note follows it, preserving the existing formatting of ordinary entries without notes. Avoid appending punctuation directly to URLs.
• Improve chapter-number formatting.
• Add an optional #:accessed field to make-bib, displayed
  immediately after the URL, when one is displayed.
  BibTeX online and webpage entries now use this field
  instead of putting access dates in their location.
  This places access dates after publication dates and
  next to the URLs they qualify, without changing the
  generic ordering of bibliography fields.
  The existing webpage-location API remains unchanged.

The improvements primarily target HTML and LaTeX. Typst receives minimal compatibility support rather than a new rendering implementation.

API, documentation and tests

• Clarify the distinction between strings, Scribble content and elements in the bibliography implementation. Location helpers may now return content or #f rather than necessarily returning an element. This is a public API change.
• Update the manuals to document the new behavior and configuration options.
• Bump scribble-lib to version 1.68.
• Add unit tests, expected-output tests and rendering tests, including regression tests for accented names, protected groups, escaped hyperlinks and multi-paragraph notes.
• Add #:accessed to make-bib; access dates are omitted when no URL is displayed, including when a DOI takes precedence.

The changes have been exercised against my book's heavily annotated bibliography and reviewed through several rounds of automated and manual testing.

Implementation was substantially assisted by GPT-5.6 Sol, with additional review by Claude Opus 5.5.

Compatibility notes

• webpage-location is removed. It was a recent addition from 1.61, I'm not sure anyone uses it directly (as opposed to through bibtex.rkt), but if they do they will have to migrate to #:accessed instead.
• bibtex-parse now keeps inner braces in raw field values
  (e.g. "The {ACM} Paper") so that protected groups survive until
  rendering; rendered output is unchanged, but code that reads
  bibdb-raw directly will now see the braces.
• BibTeX titles are no longer passed through verbatim to LaTeX;
  they go through the same reader as other fields. Math and unknown
  commands are still emitted verbatim in LaTeX output.
• Notes are now decoded like other Scribble text, so -- becomes
  an en dash and straight quotes become curly quotes.
• A purely numeric #:chapter now renders as "chapter N".
• In LaTeX output, entries may now break across pages (subject to
  the five-line minimum), and numbered bibliographies use hanging
  indentation instead of a table.
• In text output, numbered entries are no longer aligned as a table.
• Various minor bugfixes, in handling inbook, proceedings, or braces in author names.