promise microtask ordering: bare .then() chain resolves after a later combinator #9377

Symptom

promise_reaction_slot_overflow::degenerate_then_chain_survives_combinator
fails on an ordering difference — both values are correct, the sequence is
reversed:

expected: "chain: 9\nall: 9\n"
actual:   "all: 9\nchain: 9\n"

The assertion message states the intent: "a bare p.then() pass-through chain
must survive a later combinator".

Not a regression from the current release window

Confirmed with clean builds (cargo clean first — an incremental target dir
across checkouts produces false verdicts here):

So it predates the window and is independent of #9226 (the source of the two
regressions fixed in #9372/#9375).

Why it went unnoticed

cargo-test-perry shards run only in the full tier (tags / dispatch /
release branches), never on main. In the Aug-31 full tier six of eight shards
failed early — shard 7 ran 3 of 35 test binaries — so many tests, including this
one, produced no verdict at all. An absent result reads as health.

Reproduces locally

macOS/aarch64 as well as ubuntu-latest, so no Linux-specific repro is needed:

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

(build the -static wrappers first — otherwise a stale archive yields a
commit-coherence refusal rather than a test result)

Blocking

full-suite-gate fails on it, so it blocks any release cut until fixed or
triaged.