Discover in trixie intermittently freezes or crashes (SIGABRT) on JPEG-XL icons
newly introduced in Debian unstable's AppStream catalog

Steps to reproduce:

Enable unstable alongside trixie with apt-pinning, run apt update (pulls in
unstable's AppStream/DEP-11 icon catalog, including .jxl icons).
Open Discover.
Discover crashes (SIGABRT) or freezes and requires a force-close.

Root cause:

Crash/hang occurs in QImageReader::jumpToNextImage() →
QPixmapIconEngine::bestMatch(), called synchronously on the main thread during
QSGThreadedRenderLoop::polishAndSync — hence the whole UI freezes rather than
just one icon failing.
The .jxl files themselves are valid (djxl decodes them without error), so the
bug is in kimg_jxl.so's handling, not the files.
Neither libjxl0.11 nor kimageformat6-plugins had been upgraded recently; the
icon files' own mtimes show they first appeared around 2026-08-24, coinciding
with when the crash started. This points to newly-introduced .jxl content
exposing a pre-existing bug, not a package regression.
Confirmed via a second, otherwise-identical machine (same packages, no unstable
enabled, zero .jxl icons present): no crash there, ruling out CPU/core-
count/version differences.

Workaround: moving kimg_jxl.so entirely out of /usr/lib/x86_64-linux-
gnu/qt6/plugins/imageformats/ (renaming in-place to .disabled is not sufficient
— Qt loads every file in that directory regardless of extension) stops the
crash.

Note: happy to provide full GDB backtraces / strace output on request.