diag(quic-interop): summarize-matrix.sh for partial / interrupted runs

Full matrix takes long enough to be vulnerable to terminal hangs,
docker OOM, or just user CTRL-C. Per-testcase output.txt files
hold status lines we can scrape without re-running anything.

Usage:
  ./quic/interop/summarize-matrix.sh

Output:
  TESTCASE               RESULT          TIME
  --------------------   --------------- ----------
  handshake              ✓ SUCCEEDED     2.5s
  transfer               ✓ SUCCEEDED     3.1s
  multiplexing           ✓ SUCCEEDED     5.2s
  retry                  ✕ FAILED        8.3s
  ecn                    ? UNSUPPORTED   2.6s

Helps iterate when the matrix is too long to run end-to-end on
macOS Docker.

quic/interop/summarize-matrix.sh

@@ -0,0 +1,54 @@
+#!/usr/bin/env bash
+# Summarize per-testcase results for the most recent matrix run.
+# Useful when run-matrix.sh terminated early — the per-testcase
+# output.txt files have status lines we can extract.
+set -uo pipefail
+
+REPO_ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
+RUNNER_LOGS="${REPO_ROOT}/../quic-interop-runner/logs"
+
+if [[ ! -d "$RUNNER_LOGS" ]]; then
+    echo "no runner logs at $RUNNER_LOGS" >&2
+    exit 1
+fi
+
+RUN_DIR="$(ls -1dt "$RUNNER_LOGS"/run-* 2>/dev/null | head -n 1 || true)"
+if [[ -z "$RUN_DIR" ]]; then
+    echo "no run-* dirs under $RUNNER_LOGS" >&2
+    exit 1
+fi
+echo "==> run dir: $RUN_DIR"
+echo
+
+# Layout: <run>/<pair>/<testcase>/output.txt
+# The runner writes a "Test: <name> took X, status: TestResult.{SUCCEEDED|FAILED|UNSUPPORTED}"
+# line at the end of each test.
+PAIR_DIR="$(ls -1d "$RUN_DIR"/*amethyst* 2>/dev/null | head -n 1 || true)"
+if [[ -z "$PAIR_DIR" ]]; then
+    echo "no <pair> dir under $RUN_DIR" >&2
+    exit 1
+fi
+
+printf "%-22s %-15s %-10s\n" "TESTCASE" "RESULT" "TIME"
+printf "%-22s %-15s %-10s\n" "----------------------" "---------------" "----------"
+for tc_dir in "$PAIR_DIR"/*/; do
+    tc=$(basename "$tc_dir")
+    out="$tc_dir/output.txt"
+    [[ -f "$out" ]] || continue
+    # Last "Test: ... status: TestResult.X" line for this testcase.
+    line=$(grep -E "^Test: $tc took" "$out" | tail -n 1)
+    if [[ -n "$line" ]]; then
+        status=$(echo "$line" | sed -nE 's/.*TestResult\.([A-Z_]+).*/\1/p')
+        time=$(echo "$line" | sed -nE 's/.*took ([0-9.]+s).*/\1/p')
+        # Color: green=succeeded, yellow=unsupported, red=failed.
+        case "$status" in
+            SUCCEEDED) marker="✓" ;;
+            UNSUPPORTED) marker="?" ;;
+            FAILED) marker="✕" ;;
+            *) marker="·" ;;
+        esac
+        printf "%-22s %s %-13s %-10s\n" "$tc" "$marker" "$status" "$time"
+    else
+        printf "%-22s %s %-13s\n" "$tc" "·" "(no status — terminated mid-test)"
+    fi
+done