Book PDF has never built: fancyhdr header-includes clash with Eisvogel's scrlayer-scrpage (\chead already defined)

[Kartikey1306]

Build & Release Book PDF has never succeeded on this repository. The last eight runs — six failures and a cancelled tag build back to 2026-05-28 — all die at the same line:

! LaTeX Error: Command \chead already defined.
##[error]Process completed with exit code 43.

Last failing master run: 2026-09-01. The job fires only on docs/book/**, so it has not run since, and any PR that touches the book will go red on this.

Cause

.github/workflows/book-build.yml is byte-identical to eBoot's pre-fix copy (diff is empty against eBoot e152d8e), and eBoot#135 diagnosed it there:

1. The pandoc step injects \usepackage{fancyhdr} + \fancyhead/\fancyfoot through header-includes. Eisvogel builds its headers with KOMA's scrlayer-scrpage, which defines \chead; $header-includes$ sits before the template's own header block, so fancyhdr defines \chead first and the template's \newcommand fails. Both arrived in the same April commit, so the job has never been green with any Eisvogel/pandoc pair.
2. -V "author=… & …" passes a -V value verbatim into LaTeX, where & is an alignment tab.
3. Eisvogel 2.4.0 predates pandoc 3.6; the 3.x line is required, and 3.5.x needs sourcesans.sty which the runner's texlive-fonts-extra does not ship, so 3.4.0 is the last usable release.
4. tar xzf … || true hides a bad tarball until the cp after it fails on a missing file.
5. The workflow is not in its own paths: filter, so a PR that changes only the workflow cannot get the run that would prove it.

Fix

Port eBoot#135 (green twice at abf0522, eBoot-guide.pdf 48 pages — the first success in that job's history). Same five changes, one file.