Build & Release Book PDF has never produced a PDF: Eisvogel 2.4.0 fails under pandoc 3.6.1 #134

No run of Build & Release Book PDF in this repository's history has succeeded. Every run since the workflow gained its Eisvogel step (ef0dc50, 2026-04-25) is failure or cancelled — the May release runs (v1.4.0 1bc7641, v1.5.0 fa2b3b1) through the September ones (master 84bbef0 on 09-08, and every fix/eboot-test-issue push before it). Logs older than the retention window have expired; every run whose log is still readable (09-05 onward) dies the same way:

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

book-build.yml pins PANDOC_VERSION: "3.6.1" (installed from the release .deb) and fetches Eisvogel 2.4.0, which predates pandoc 3.6; the Eisvogel 3.x line merges pandoc's default template through 3.6.1 and later and states pandoc 3 compatibility.

Two further defects in the same step: tar xzf … 2>/dev/null || true is fail-open on the step that is breaking (a missing or reshaped tarball fails silently and the diagnostic lands on the cp after it), and the job only triggers on docs/book/**, so a change to the workflow itself cannot get the run that would prove it.

ebuild carries the identical step at its book-build.yml:100 and is red the same way; eos is green only because its copy installs apt's pandoc 2.9 and no Eisvogel at all. Currently visible on eBoot #133, the only open PR touching docs/.