[Bug]: update deadlocks because of reaper #6733

▎ During a 35-ref flatpak update -y --noninteractive, the apply_extra step of fm.reaper.Reaper (extra-data) hangs forever. The extraction (tar xf reaper.tar.xz → xz -d; freedesktop 25.08 runtime, tar 1.35 / xz 5.8.3) sleeps at 0 CPU from the first second — a 13.6 MB tarball that should take ~1s. The tarball is complete (13,599,360 bytes, matches the manifest's extra-data size, sha256 already verified), so this isn't a truncated download.
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▎ Since the hung process holds the system installation lock, every other flatpak install/update on the machine blocks. One wedged run went unnoticed for 6 weeks (uupd.service: Consumed 15.705s CPU time over 3w 4d 18h ... wall clock time).

 State at hang time:
▎
▎ uupd(4020370)─flatpak update -y --noninteractive(4021653)
▎   └─bwrap(4022328)─bwrap(4022329)─sh /app/bin/apply_extra(4022330)
▎       └─tar xf reaper.tar.xz -C /app/extra/export/share(4022332)
▎           └─xz -d(4022333)