fix(quic-interop): summarize-matrix picks newest run dir, not sibling .stdout.log

`ls -1dt run-*` returns both the per-run directories AND the
`.stdout.log` files run-matrix.sh tees alongside them, interleaved by
mtime. `head -n 1` could land on a `.stdout.log` regular file, after
which the rest of the script trying to walk subdirectories under it
silently produced empty output ("no <pair> dir under …"). Walk the
listing instead and pick the first entry that is actually a directory.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

quic/interop/summarize-matrix.sh

@@ -16,7 +16,13 @@ if [[ ! -d "$RUNNER_LOGS" ]]; then
     exit 1
 fi
 
-RUN_DIR="$(ls -1dt "$RUNNER_LOGS"/run-* 2>/dev/null | head -n 1 || true)"
+RUN_DIR=""
+while IFS= read -r candidate; do
+    if [[ -d "$candidate" ]]; then
+        RUN_DIR="$candidate"
+        break
+    fi
+done < <(ls -1dt "$RUNNER_LOGS"/run-* 2>/dev/null)
 if [[ -z "$RUN_DIR" ]]; then
     echo "no run-* dirs under $RUNNER_LOGS" >&2
     exit 1