diag(quic-interop): tee runner stdout to sibling log + summarize reads it

Two paired changes:

(1) run-matrix.sh now tees the runner's full stdout (pre-grep-
    filter) to <RUN_LOG_DIR>.stdout.log — sibling rather than
    inside the log dir because run.py refuses to start if its own
    --log-dir already exists.

(2) summarize-matrix.sh checks that file FIRST when searching for
    'Test: X took Y, status:' lines — it has the authoritative
    runner output that wasn't being saved before.

Old runs (without the tee) fall back to qlog inspection.

quic/interop/summarize-matrix.sh

@@ -67,6 +67,9 @@ infer_status_from_artifacts() {
 # and the working dir we were invoked from.
 SEARCH_FILES=()
 while IFS= read -r f; do SEARCH_FILES+=("$f"); done < <(
+    # run-matrix.sh tees the runner's stdout to <RUN_DIR>.stdout.log —
+    # check that first since it has the authoritative status lines.
+    [[ -f "${RUN_DIR}.stdout.log" ]] && echo "${RUN_DIR}.stdout.log"
     find "$PAIR_DIR" -maxdepth 3 -name 'output.txt' -type f 2>/dev/null
     find "$RUN_DIR" -maxdepth 1 -name '*.log' -type f 2>/dev/null
     find "$RUNNER_LOGS" -maxdepth 1 -name 'run-*.log' -type f 2>/dev/null