chore(quic-interop): inspect-multiplexing.sh post-mortem helper

Pulls the diagnostics needed to localize a multiplexing failure:
  - tail client/log.txt (per-stream errors, final outcome)
  - tail server/log.txt (peer's CONNECTION_CLOSE reason)
  - tail client qlog (frame-level event sequence)
  - count downloaded files (how many of N actually finished)
  - qlog event-type histogram (e.g. "lots of packet_dropped",
    "no stream_state_updated past t=30s", etc.)

Resolves the most recent run-* dir under ../quic-interop-runner/logs
automatically — no need to copy-paste paths after every run.

https://claude.ai/code/session_01HcvfQq1ttPV9PkRoJb4nyT

quic/interop/inspect-multiplexing.sh

@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+# Pull the post-mortem diagnostics for the most recent multiplexing run.
+# Runs from anywhere; resolves logs relative to the runner clone path
+# you've been using (../quic-interop-runner from this repo).
+set -euo 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 $RUNNER_LOGS" >&2
+    exit 1
+fi
+
+# Most recent run dir.
+RUN_DIR="$(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: $RUN_DIR"
+
+# Multiplexing case dir (server/client pair). Glob the only one matching.
+CASE_DIR="$(ls -1d "$RUN_DIR"/multiplexing/*amethyst* 2>/dev/null | head -n 1 || true)"
+if [[ -z "$CASE_DIR" ]]; then
+    echo "no multiplexing/*amethyst* dir; tree under run:" >&2
+    find "$RUN_DIR" -maxdepth 3 -type d >&2
+    exit 1
+fi
+echo "==> case dir: $CASE_DIR"
+
+echo
+echo "=============== client/log.txt (tail) ==============="
+tail -n 200 "$CASE_DIR"/client/log.txt 2>/dev/null \
+    || echo "(no client/log.txt)"
+
+echo
+echo "=============== server/log.txt (tail) ==============="
+tail -n 200 "$CASE_DIR"/server/log.txt 2>/dev/null \
+    || echo "(no server/log.txt)"
+
+echo
+echo "=============== client qlog (tail of last 100 events) ==============="
+QLOG="$(ls -1 "$CASE_DIR"/client/*.sqlog "$CASE_DIR"/client/*.qlog 2>/dev/null | head -n 1 || true)"
+if [[ -n "$QLOG" ]]; then
+    echo "(file: $QLOG)"
+    tail -n 100 "$QLOG"
+else
+    echo "(no qlog under $CASE_DIR/client)"
+fi
+
+echo
+echo "=============== /downloads contents ==============="
+DL_DIR="$CASE_DIR/download"
+[[ -d "$DL_DIR" ]] || DL_DIR="$CASE_DIR/client/downloads"
+if [[ -d "$DL_DIR" ]]; then
+    DOWNLOADED="$(find "$DL_DIR" -type f | wc -l | tr -d ' ')"
+    echo "files downloaded: $DOWNLOADED"
+    echo "first 5:"
+    find "$DL_DIR" -type f | head -n 5
+    echo "last 5:"
+    find "$DL_DIR" -type f | tail -n 5
+else
+    echo "(no downloads dir found; checked $CASE_DIR/download and $CASE_DIR/client/downloads)"
+fi
+
+echo
+echo "=============== qlog event-type histogram ==============="
+if [[ -n "${QLOG:-}" ]]; then
+    # Each NDJSON line carries `"name":"transport:packet_sent"` etc.
+    # Histogram the names so you see "many packet_dropped" or "lots of
+    # ack_received but stream_state_updated tapered off".
+    grep -oE '"name":"[^"]+"' "$QLOG" | sort | uniq -c | sort -rn | head -n 20
+fi