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/src/main/kotlin/com/vitorpamplona/quic/interop/runner/InteropClient.kt

@@ -141,7 +141,12 @@ fun main() {
             "handshake", "chacha20", "handshakeloss",
             "transfer", "http3", "multiplexing",
             "transferloss", "transfercorruption", "longrtt", "goodput", "crosstraffic",
-            "retry", "ipv6", "versionnegotiation",
+            "retry", "ipv6",
+            // NOTE: the runner does NOT have a `versionnegotiation` testcase
+            // (its Available list excludes it). The :quic VN-handling code
+            // (applyVersionNegotiation, FORCE_VERSION_NEGOTIATION constant)
+            // stays as defensive support for any server that decides to
+            // send a VN packet at us, but no testcase exercises it directly.
             -> {
                 runTransferTest(
                     requests = requests,