fix(engine): retry passes rsync exit 23, a dangling symlink under -L

[jshvn]

What

Every ctan sync since 2026-09-13 07:43 UTC fails at list with rsync exit 23: four tugboat.cmn symlinks under macros/latex209/contrib/ and obsolete/ lost their target upstream between the 06:43 and 07:43 runs, and -L reports each as symlink has no referent. The rest of the listing is intact (the 06:43 run had none of these lines and listed 512,375 objects), but retry passed only 0 and 24, so the pipeline died six runs in a row.

• engines/rsync.yml: retry treats 23 like 24. For list it is a dangling symlink skipped, and LIST_FLOOR is what catches a listing cut short. For fetch it is a path that could not be read, which never lands in staging, so merge drops it from the state exactly as it does a vanished one. For prepare a missing control file fails the next command. A comment on list says why 23 is passed there.
• examples/rsync: the offline check gains task retry CMD='exit 23', which fails on main; render.txt changes by the three renderings of the case line.
• README: the retry row.

Storage and operations

None. No verb writes anything differently.

Test plan

• Reproduced in the rsync image against rsync://rsync.dante.ctan.org/CTAN/macros/latex209/contrib/tugboat/: -rL --list-only prints the referent line, lists the two real files, exits 23; -r lists the link and exits 0
• examples/rsync: task run -- task offline fails on main with the new line and passes here; task check passes after task render-update
• CI green
• After merge: tag v2.1.1, which rebuilds the images and moves v2; the next hourly ctan run is the live check
• After the tag: task render-update in ctan and tlnet, three lines each