feat(quic-interop): wire peer-uni-stream drainer + versionnegotiation

Two integrations from overnight agents:

1. agent C — peer-uni-stream drainer for the H3 multiplexing fix.
   Http3GetClient.init() now takes a CoroutineScope and calls
   conn.drainPeerInitiatedUniStreamsIntoBlackHole(scope) to consume +
   discard the server's three uni streams (control, qpack-encoder,
   qpack-decoder). RFC 9114 §6.2 mandates the client read these;
   pre-fix their bytes accumulated in 64-chunk per-stream channels
   until parser overflow tore down the connection with INTERNAL_ERROR
   ~4.5s into a multiplexing run.

2. agent A — versionnegotiation testcase wired into dispatch.
   Adds the testcase to the runTransferTest route and threads
   QuicVersion.FORCE_VERSION_NEGOTIATION as the initial version when
   testcase=='versionnegotiation'. The QuicConnection's RFC 9000 §6
   VN flow (applyVersionNegotiation) takes over from there, retrying
   with v1 once the server replies with its supported list.

https://claude.ai/code/session_01HcvfQq1ttPV9PkRoJb4nyT

quic/interop/src/main/kotlin/com/vitorpamplona/quic/interop/runner/Http3GetClient.kt

@@ -22,6 +22,7 @@ package com.vitorpamplona.quic.interop.runner
 
 import com.vitorpamplona.quic.QuicWriter
 import com.vitorpamplona.quic.connection.QuicConnection
+import com.vitorpamplona.quic.connection.drainPeerInitiatedUniStreamsIntoBlackHole
 import com.vitorpamplona.quic.http3.Http3Frame
 import com.vitorpamplona.quic.http3.Http3FrameReader
 import com.vitorpamplona.quic.http3.Http3FrameType
@@ -29,6 +30,7 @@ import com.vitorpamplona.quic.http3.Http3Settings
 import com.vitorpamplona.quic.http3.Http3StreamType
 import com.vitorpamplona.quic.qpack.QpackDecoder
 import com.vitorpamplona.quic.qpack.QpackEncoder
+import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.flow.collect
 
 /** Common shape for the two interop GET clients (HTTP/3 and HQ-interop). */
@@ -59,7 +61,7 @@ data class GetResponse(
 class Http3GetClient(
     private val conn: QuicConnection,
 ) : GetClient {
-    suspend fun init() {
+    suspend fun init(scope: CoroutineScope) {
         // Control stream: type-0x00 prefix followed by a SETTINGS frame
         // (empty body is legal — RFC 9114 §7.2.4).
         val control = conn.openUniStream()
@@ -81,6 +83,16 @@ class Http3GetClient(
         val w3 = QuicWriter()
         w3.writeVarint(Http3StreamType.QPACK_DECODER)
         qpackDec.send.enqueue(w3.toByteArray())
+
+        // RFC 9114 §6.2: the server opens its own three uni streams
+        // (control, qpack encoder, qpack decoder). Their bytes accumulate
+        // in per-stream `incomingChannel`s (capacity 64); without an
+        // active consumer the channel saturates and `:quic` tears down
+        // the connection with INTERNAL_ERROR. We don't actually use the
+        // dynamic table or care about the server's settings, so drain
+        // and discard. Without this, multiplexing testcase fails after
+        // ~4.5s with "consumer overflowed" tear-down.
+        conn.drainPeerInitiatedUniStreamsIntoBlackHole(scope)
     }
 
     /**

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

@@ -89,6 +89,16 @@ fun main() {
             else -> null
         }
 
+    // For the versionnegotiation testcase the runner expects us to send
+    // an Initial advertising a version the server doesn't support, then
+    // honor its VN response by retrying with v1. agent A's
+    // QuicVersion.FORCE_VERSION_NEGOTIATION drives that flow.
+    val initialVersion =
+        when (testcase) {
+            "versionnegotiation" -> com.vitorpamplona.quic.packet.QuicVersion.FORCE_VERSION_NEGOTIATION
+            else -> com.vitorpamplona.quic.packet.QuicVersion.V1
+        }
+
     // ALPN selection. Different servers configure different ALPNs PER
     // testcase, with no consistent convention:
     //   - quic-go-qns         — strictly hq-interop for non-http3 tests
@@ -131,13 +141,14 @@ fun main() {
             "handshake", "chacha20", "handshakeloss",
             "transfer", "http3", "multiplexing",
             "transferloss", "transfercorruption", "longrtt", "goodput", "crosstraffic",
-            "retry", "ipv6",
+            "retry", "ipv6", "versionnegotiation",
             -> {
                 runTransferTest(
                     requests = requests,
                     downloadsDir = downloadsDir,
                     cipherSuites = cipherSuites,
                     offeredAlpns = offeredAlpns,
+                    initialVersion = initialVersion,
                     keyLogPath = keyLogPath,
                     parallel = (testcase == "multiplexing"),
                 )
@@ -168,6 +179,7 @@ private fun runTransferTest(
     downloadsDir: File,
     cipherSuites: IntArray?,
     offeredAlpns: List<Alpn>,
+    initialVersion: Int,
     keyLogPath: String?,
     parallel: Boolean,
 ): Int {
@@ -204,6 +216,7 @@ private fun runTransferTest(
                     config = QuicConnectionConfig(),
                     tlsCertificateValidator = PermissiveCertificateValidator(),
                     alpnList = offeredAlpns.map { it.wireBytes },
+                    initialVersion = initialVersion,
                     cipherSuites =
                         cipherSuites
                             ?: intArrayOf(
@@ -235,7 +248,7 @@ private fun runTransferTest(
             val client: GetClient =
                 when (negotiated) {
                     "h3" -> {
-                        Http3GetClient(conn).also { it.init() }
+                        Http3GetClient(conn).also { it.init(scope) }
                     }
 
                     "hq-interop" -> {