fix(quic-interop): drop versionnegotiation; document concurrency limit

Two corrections from the user's just-completed run:

1. The runner does not have a 'versionnegotiation' testcase. Available
   list output: handshake, transfer, longrtt, chacha20, multiplexing,
   retry, resumption, zerortt, http3, blackhole, keyupdate, ecn,
   amplificationlimit, handshakeloss, transferloss, handshakecorruption,
   transfercorruption, ipv6, v2, rebind-port, rebind-addr,
   connectionmigration. (`v2` exists but tests QUIC v2 protocol
   support — we're v1-only, so it would correctly fail.)

   The :quic-side VN code from agent A stays as defensive support for
   any future server that throws a VN at us; just no testcase wires
   it up.

2. run-matrix.sh is NOT safe to run concurrently. The runner's
   docker-compose.yml hardcodes container_name: sim/server/client —
   Docker enforces those globally regardless of COMPOSE_PROJECT_NAME.
   Documented the limitation + the recommended sequential loop in
   the script's comments.

https://claude.ai/code/session_01HcvfQq1ttPV9PkRoJb4nyT

quic/interop/run-matrix.sh

@@ -112,6 +112,15 @@ fi
 # per-invocation subdirectory under $LOG_DIR. The parent must exist; the
 # child must not. Tail of `ls -t "$LOG_DIR" | head -1` finds the latest.
 #
+# NOTE: this script is NOT safe to run concurrently against itself.
+# quic-interop-runner's docker-compose.yml hardcodes `container_name:
+# sim/server/client`, which Docker enforces globally regardless of
+# COMPOSE_PROJECT_NAME. Two simultaneous invocations collide on
+# `docker create container "sim"`. Run sequentially:
+#   for peer in aioquic picoquic quic-go; do
+#       quic/interop/run-matrix.sh -s $peer -t handshake,chacha20,...
+#   done
+#
 # Output filter: by default we drop the runner / container boilerplate
 # (interface checksum offload toggles, route setup, container lifecycle,
 # the long Command: WAITFORSERVER=... line, the platform-mismatch warning