Bug#1139505: JadeTeX issue: Undefined control sequence

Hi,

tig maintainer here, who stumbled over the same issue. I've verified the FTBFS 
for tig is solved by monkeypatching the installed file and running `fmtutil-sys 
--byfmt pdfjadetex`. I tried verifying it rebuilding texlive-extra, but I don't 
really have the extra disk space (> 15 GiB) right now. The patch is this one-liner:

--- a/texmf-dist/tex/jadetex/base/jadetex.ltx
+++ b/texmf-dist/tex/jadetex/base/jadetex.ltx
@@ -1309,6 +1309,7 @@
    \global\@maxdepth\maxdepth
+  \@expl@sys@load@backend@@
    \@kernel@before@begindocument
    \UseOneTimeHook{begindocument}%

\JadeBeginDocument somewhat resembles \document (latex.ltx:9632), and the latter 
has the missing line.

How it was introduced in Debian: hyperref commit "extend \XeTeXLinkBox to 
dvipdfmx backend" added an \AddToHook{begindocument} that reads 
\c_sys_backend_str [0]. This commit ended up in hyperref 7.01q, which in turn 
ended in Debian with texlive-base 2026.20260527-1. The bug in jadetex seems to 
be much, much older (< 2020), it's just the first time it was triggered by a 
package.

[0] 
https://github.com/latex3/hyperref/commit/e6d909444e19efb64a3db5d24231474828f79f51

Greets, Lee