test(e2e-stress): improve stress phase observability

Send the "stress matrix invalid" message to stderr and echo a WARN on
stress_run.py soft-failure, matching the script's existing soft-fail
echo convention.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Test/E2E/chain/run_chain_e2e.sh

@@ -284,10 +284,11 @@ if [ "${STRESS}" -eq 1 ]; then
     echo ""
     echo "── Stress matrix ──"
     mkdir -p "${STRESS_OUT}"
-    ${PY} "${SCRIPT_DIR}/stress.py" >/dev/null || { echo "stress matrix invalid"; exit 1; }
+    ${PY} "${SCRIPT_DIR}/stress.py" >/dev/null || { echo "stress matrix invalid" >&2; exit 1; }
     ${PY} "${SCRIPT_DIR}/stress_run.py" \
         --marte "${MARTE_APP}" --hub "${STREAMHUB_EX}" --client "${CLIENT}" \
-        --work "${WORK}" --out "${STRESS_OUT}" || true
+        --work "${WORK}" --out "${STRESS_OUT}" \
+        || { echo "WARN: stress_run.py failed, continuing" >&2; true; }
 fi
 
 # ── Consolidated report data (+ history/regression + trend plots) ────────────