fix(test): bound race-round barrier spin so a missing release flag fails instead of hanging

[danikdanik]

What

_race_round() in test_windows_portability.sh held 10 forked racers on an unbounded busy-spin (while [ ! -f "$flag" ]; do :; done). If the release flag can never appear — the mktemp workdir was swept mid-round, : > "$flag" failed, or TMP was empty — every racer burns ~65% CPU forever while wait parks the suite.

Why it matters

Observed on macOS (bash 3.2.57): a single conformance run parked for 16+ hours with ~7 cores of pure busy-loop, because conformance.sh invokes suites with no timeout and the test has no bound of its own. The failure is silent — nothing in the output says the suite wedged.

How

• Racers now concede when the workdir is gone or a 15s SECONDS deadline hits — a broken barrier reports FAIL instead of hanging.
• _race_round returns a distinct ERR on barrier failure (flag create fails, or won/flag vanish mid-round), and both callers stop on the first broken round — no more 40×15s slow-burn on a persistent failure, and 0 winners can no longer masquerade as a "control caught non-atomic claim" pass.
• wait $pids instead of bare wait; true > instead of : > so a redirect failure can't exit POSIX shells before || runs; entry guard for empty/missing TMP.

Repro / verification

• Healthy run (unchanged): passed=20 failed=0 in ~15s, control still catches 20/20.
• Fault injection (the incident): delete the mktemp workdir while racers spin.
  • Before: parent parked at wait in S, 10 children at ~65% CPU in R, indefinitely.
  • After: FAIL: race barrier broken — release flag never appeared (round N), suite exits rc=1 in ~2s.

# minimal reproducer of the mechanism (pre-fix)
TMP="$(mktemp -d)"; flag="$TMP/go.$$"
for i in $(seq 10); do ( while [ ! -f "$flag" ]; do :; done ) & done
sleep 0.3; rm -rf "$TMP"; : > "$flag" 2>/dev/null   # create fails silently
wait   # parks forever; racers spin forever

Residual (out of scope): conformance.sh still has no per-suite timeout, so a suite that wedges outside a bounded spin can still park the gate — worth a follow-up.