fix(quic): batch openBidiStream under one lock hold for multiplexing

Diagnosis from yet another qlog round: streams/packet still ~1 even
with the prepareRequest/awaitResponse split. Root cause: openBidiStream
is suspend due to lock.withLock, and each call releases the lock between
iterations. The send loop is queued on the lock; it grabs it the moment
we release, drains the one stream of data we just enqueued, and the
next prepareRequest call has to re-acquire after the send loop releases.
Net: one stream per drain per packet, same useless coalescing as before.

Fix is structural:
  - QuicConnection.openBidiStreamLocked() — public, lock-not-acquired
    version of openBidiStream. Caller MUST hold conn.lock.
  - GetClient.prepareRequests(authority, paths) — batch API that
    holds conn.lock once, opens + enqueues all N streams in a single
    critical section, releases. Send loop can't interject; when it
    next drains it sees ALL N streams' data ready and packs them
    into coalesced packets.
  - Http3GetClient + HqInteropGetClient: implement prepareRequests
    using openBidiStreamLocked under conn.lock.withLock { ... }.
  - InteropClient's chunked-multiplex loop: uses prepareRequests
    (batch) instead of N x prepareRequest.

Single-stream paths still use prepareRequest / get(); behavior unchanged.

The fundamental architectural improvement (per-stream / per-level lock
split, or actor-model dispatch) is a follow-up; this commit gets us
the throughput we need from the existing single-mutex shape by holding
the lock for the full chunk's worth of work.

https://claude.ai/code/session_01HcvfQq1ttPV9PkRoJb4nyT

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

@@ -23,6 +23,7 @@ package com.vitorpamplona.quic.interop.runner
 import com.vitorpamplona.quic.connection.QuicConnection
 import com.vitorpamplona.quic.connection.QuicConnectionDriver
 import kotlinx.coroutines.flow.toList
+import kotlinx.coroutines.sync.withLock
 
 /**
  * HQ-interop (HTTP/0.9 over QUIC) GET client. quic-interop-runner convention
@@ -52,6 +53,20 @@ class HqInteropGetClient(
         return HqRequestHandle(stream)
     }
 
+    override suspend fun prepareRequests(
+        @Suppress("UNUSED_PARAMETER") authority: String,
+        paths: List<String>,
+    ): List<RequestHandle> =
+        conn.lock.withLock {
+            paths.map { path ->
+                val stream = conn.openBidiStreamLocked()
+                val request = "GET $path\r\n".encodeToByteArray()
+                stream.send.enqueue(request)
+                stream.send.finish()
+                HqRequestHandle(stream)
+            }
+        }
+
     override suspend fun awaitResponse(handle: RequestHandle): GetResponse {
         val stream = (handle as HqRequestHandle).stream
         val chunks = stream.incoming.toList()

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

@@ -33,6 +33,7 @@ import com.vitorpamplona.quic.qpack.QpackDecoder
 import com.vitorpamplona.quic.qpack.QpackEncoder
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.flow.collect
+import kotlinx.coroutines.sync.withLock
 
 /** Common shape for the two interop GET clients (HTTP/3 and HQ-interop).
  *
@@ -49,6 +50,23 @@ interface GetClient {
         path: String,
     ): RequestHandle
 
+    /**
+     * Atomically open + enqueue + FIN N streams under a single hold of
+     * the connection lock. The send loop cannot interject between
+     * opens, so when it next drains it sees ALL N streams' data ready
+     * and packs them into coalesced packets instead of emitting one
+     * tiny packet per stream.
+     *
+     * Without this, the equivalent serial loop of [prepareRequest]
+     * yields between calls (lock release → send loop wakes → drains
+     * one stream → next prepareRequest acquires...) and we send one
+     * stream per packet — what cratered the multiplexing testcase.
+     */
+    suspend fun prepareRequests(
+        authority: String,
+        paths: List<String>,
+    ): List<RequestHandle>
+
     /** Suspend until the server FINs the response stream associated with
      *  [handle]. Returns the assembled response. */
     suspend fun awaitResponse(handle: RequestHandle): GetResponse
@@ -137,6 +155,19 @@ class Http3GetClient(
         return Http3RequestHandle(stream)
     }
 
+    override suspend fun prepareRequests(
+        authority: String,
+        paths: List<String>,
+    ): List<RequestHandle> =
+        conn.lock.withLock {
+            paths.map { path ->
+                val stream = conn.openBidiStreamLocked()
+                stream.send.enqueue(encodeRequest(authority, path))
+                stream.send.finish()
+                Http3RequestHandle(stream)
+            }
+        }
+
     override suspend fun awaitResponse(handle: RequestHandle): GetResponse {
         val stream = (handle as Http3RequestHandle).stream
         val reader = Http3FrameReader()

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

@@ -349,14 +349,16 @@ private fun runTransferTest(
                             // by batching enqueues + single wake.
                             val collected = mutableListOf<Pair<URI, GetResponse>>()
                             urls.chunked(MULTIPLEX_PARALLELISM).forEach { chunk ->
-                                val prepared =
-                                    chunk.map { url ->
-                                        url to client.prepareRequest(authority, url.path)
-                                    }
+                                // Single lock-held batch open + enqueue.
+                                // Without this, openBidiStream's per-call
+                                // lock acquire / release lets the send loop
+                                // interject between opens and drain one
+                                // stream per packet.
+                                val handles = client.prepareRequests(authority, chunk.map { it.path })
                                 driver.wakeup()
                                 coroutineScope {
                                     val deferreds =
-                                        prepared.map { (url, handle) ->
+                                        chunk.zip(handles).map { (url, handle) ->
                                             async {
                                                 val resp =
                                                     withTimeoutOrNull(PER_STREAM_TIMEOUT_SEC * 1_000L) {