Catalog hint placeholders survive a rebuild that carries only args

What — A catalog hint whose template interpolates a field cannot be re-rendered
by a rebuilder that carries only args, because the field is not there.
How — type(w)(*w.args) on a warning whose user_hint is "{guess} …"
renders the placeholder literally. pickle and copy.deepcopy are
unaffected: they carry extra.
Why — Under released pytest-xdist a warning crossing from a worker to the
controller is rebuilt from args alone. ArgDigest's
DigestNotDigestedWarning shows 'unknown' where the argument name
should be. Serial runs, scripts, notebooks and services are unaffected.
Record — devguide/pending_bugs/catalog_warnings_re_rendered_under_xdist.md

Blocked on pytest-dev/pytest-xdist#1372, which proposes transferring the warning
state. No class shape fixes this: the state has to travel.

The larger defect this document opened with — a catalog warning rendering as its
own template applied to its own output — was resolved in 0.13.0 and is guarded by
tests/test_catalog_instance_round_trip.py. This issue tracks only the residue.

Start with devguide/pending_bugs/catalog_warnings_re_rendered_under_xdist.md and the pytest-dev/pytest-xdist#1372 blocker. Compare the remaining behavior with tests/test_catalog_instance_round_trip.py, which guards the resolved adjacent defect. The issue is done when the warning state can cross the worker boundary and the catalog hint no longer renders its placeholder literally.