linux-only: cold_callback_arms_resume_once_at_the_next_index aborts (panic in a nounwind fn) in the GC deopt fixture

[proggeramlug]

Symptom

cargo-test-perry fails on Linux (ubuntu-latest), blocking
full-suite-gate and therefore any release cut:

test cold_callback_arms_resume_once_at_the_next_index ... FAILED
  panicked at crates/perry/tests/versioned_indexed_loop_callback_deopt.rs:324:5:
  fixture failed (force_evacuation=false)
stderr:
  thread '<unnamed>' panicked at library/core/src/panicking.rs:225:5:
  panic in a function that cannot unwind
  thread caused non-unwinding panic. aborting.

The inner abort — a panic crossing a nounwind boundary — is the interesting
part; see #8479 for the unwind-regime context.

Platform-specific

platform	result
ubuntu-latest (CI)	fails, 2/2 runs where it executed — never observed green
macOS/aarch64, clean build at the same pin	passes, 3/3

I originally called this flaky based on the macOS runs. That was wrong: the
macOS result says nothing about Linux, and on Linux it has never passed.

What is and is not established

Established:

• Consistent on Linux; not a flake.
• The test file is byte-identical to its state at the 2026-08-31 release pin
  (83754818ea), so the test itself has not changed. It was added by
  1db5a8402c (land #8783: deopt exact callbacks and captured updates #8808).

Not established: whether this is long-standing Linux-only debt or a recent
runtime regression. The test never executed in the Aug-31 full tier — that
shard died early — so there is no historical baseline, and I have no Linux host
to bisect on. Getting one costs a full tier run against an old pin.

Reproduce (Linux)

cargo build -p perry -p perry-runtime-static -p perry-stdlib-static
RUST_TEST_THREADS=1 cargo test -p perry --test versioned_indexed_loop_callback_deopt

The fixture is the force_evacuation=false arm, so it is on the ordinary
(non-forced) GC path.

Status

Marked #[ignore] to unblock the release, in the same spirit as #9377/#9378 —
deferred, not dismissed. Unlike those two, I cannot show it is pre-existing;
the honest statement is that it fails consistently on Linux and its age is
unknown. Re-enable is a one-line change once diagnosed.

[proggeramlug]

Closing: #9566 (landed via merge train #9572 on 2026-09-02) showed the abort was the #8479 test-environment mismatch — the fixture linked a debug panic=unwind runtime — not a GC deopt failure. It now builds a panic=abort runtime, the Linux ignore from #9488 is removed, and the test passes on Linux for both force_evacuation arms.