fix(gate): check_vm_parity portable timeout — macOS runner ships no timeout

Once the surface_marker false-positive stopped short-circuiting ci_gate.sh
before its step 3, the nightly macOS full gate surfaced 382 tree-walk e2e
"failures", every one with an empty diff. Root cause: check_vm_parity.sh
wrapped each e2e in raw `timeout 180 bash "$t"`, but hosted macOS runners ship
neither GNU `timeout` nor coreutils `gtimeout` → "command not found" (exit 127)
→ each e2e counted as a FAIL whose output matched no FAIL/error pattern (hence
the empty `()`). corpus passed because check_corpus_regression already uses the
run_bounded fallback. This was the last raw `timeout` caller missed by commit
264804b's sweep; ported to the same run_bounded helper (timeout → gtimeout →
unbounded). Reproduced the runner condition locally (timeout/gtimeout absent):
raw timeout → exit 127; run_bounded → exit 0, e2e green.

Smell-audited: 1: test-harness portability, no runtime logic change; verified via a clean-PATH macOS-runner simulation + smoke green.

Author: chaploud

scripts/check_vm_parity.sh

@@ -35,6 +35,19 @@ SKIPS=(
 restore() { zig build -Dwasm -Doptimize=ReleaseSafe >/dev/null 2>&1 || true; }
 trap restore EXIT
 
+# Portable bounded run: GNU `timeout`, else macOS coreutils `gtimeout`, else
+# unbounded. Hosted macOS runners ship NEITHER — a raw `timeout` there errors
+# with "command not found" (exit 127), which this script would have misread as
+# a tree-walk FAIL for every e2e (empty diff, since 127's text matches no
+# FAIL/error). This is the same helper the e2e scripts + check_corpus_regression
+# use; check_vm_parity was the last raw caller missed by commit 264804b7's sweep.
+run_bounded() {
+    local secs="$1"; shift
+    if command -v timeout >/dev/null 2>&1; then timeout "$secs" "$@"
+    elif command -v gtimeout >/dev/null 2>&1; then gtimeout "$secs" "$@"
+    else "$@"; fi
+}
+
 echo "check_vm_parity: building -Dbackend=tree_walk -Doptimize=ReleaseSafe…"
 if ! zig build -Dwasm -Doptimize=ReleaseSafe -Dbackend=tree_walk >/tmp/vmp_build.txt 2>&1; then
     echo "check_vm_parity: tree-walk BUILD FAILED (see /tmp/vmp_build.txt)"; exit 1
@@ -60,7 +73,7 @@ for t in test/e2e/*.sh; do
         echo "  SKIP (default-backend)    : $name"
         continue
     fi
-    if timeout 180 bash "$t" >/tmp/vmp_e2e.txt 2>&1; then
+    if run_bounded 180 bash "$t" >/tmp/vmp_e2e.txt 2>&1; then
         : # green — stay quiet (the suite is ~250 steps)
     else
         echo "  FAIL on tree-walk         : $name  ($(grep -iE 'FAIL|error' /tmp/vmp_e2e.txt | head -1 | cut -c1-100))"