fix(crossref): apply appendix-title to appendix cross-references

[cderv]

When a book sets crossref.appendix-title, the appendix chapter heading uses it (e.g. Whatever A: Summary in HTML), but cross-references to that appendix render the default Appendix N prefix in PDF — the configured title is ignored. The language: crossref-apx-prefix workaround from the issue only papers over the symptom and does nothing for appendix-delim.

Root Cause

The appendix cross-reference prefix is built from language.crossref-apx-prefix alone, never crossref.appendix-title. Two independent code paths build it:

1. Lua refPrefix (src/resources/filters/crossref/format.lua) — PDF, and HTML same-file references.
2. TS formatCrossref (src/project/types/book/book-crossrefs.ts) — HTML cross-file references resolved at the book post-render stage.

Fix

Both paths now prefer crossref.appendix-title when set, falling back to the language string — mirroring the precedence formatChapterTitle already uses for the appendix heading. Behavior is unchanged when appendix-title is unset.

The HTML cross-file path only resolves after a full project render, so its smoke-all test uses render-project: true. The project pre-render builds only declared formats, so the fixture declares format: html; this render-project behavior is now documented in the testing rules and llm-docs.

The PDF/LaTeX appendix chapter heading itself (as opposed to cross-references) still ignores appendix-title/appendix-delim; matching the HTML Whatever A: Summary heading in LaTeX needs \appendixname/KOMA work and is tracked separately in #14616.

Test Plan

• Book PDF: an @sec- reference to an appendix shows See Whatever A, not See Appendix A
• Book HTML: a cross-file appendix reference from a numbered chapter shows the configured title
• With appendix-title unset: appendix cross-references unchanged (default Appendix N)

Fixes #11772

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[cderv]

Closing this — on re-reading the issue, it solves the wrong thing. Reviewing the code it does not seem right to use appendix-title as the prefix.

The reported problem is the appendix chapter heading in PDF: HTML shows Attachment A: Details, PDF shows A Details. This PR instead changes the appendix cross-reference prefix, which was already consistent between HTML and PDF and isn't what the issue is about. crossref.appendix-title is the heading label; the cross-reference prefix is the separate localized crossref-apx-prefix, so routing the title into cross-references also risks overriding the localized prefix under lang:.

I'll reopen work focused on the PDF appendix heading itself.