fix(quic-interop): explicitly advertise QPACK_MAX_TABLE_CAPACITY=0

aioquic multiplexing diagnosis: runner asked for 1999 files, aioquic
processed 1371 GET requests, our endpoint wrote ONLY 1 file to
/downloads. The connection stayed healthy throughout (qlog shows
STREAM frames flowing both ways at t=58s). Server response volume
indicates each request got a 200 response.

The bug: our QpackDecoder is literal-only (no dynamic table). When
aioquic primes its dynamic table after a few requests and switches
to dynamic-table references in subsequent response HEADERS, our
decoder silently mis-parses — status comes back 0, file not written.
Empty-SETTINGS gives aioquic the spec default (also 0) but it apparently
doesn't strictly enforce: it still emits dynamic-refs anyway.

Fix: explicitly advertise QPACK_MAX_TABLE_CAPACITY=0 +
QPACK_BLOCKED_STREAMS=0 in our SETTINGS. Forces aioquic onto the
literal-only QPACK path that our decoder handles correctly.

A proper fix would be to implement QPACK dynamic-table support in our
decoder + read the server's encoder stream; that's its own project.
For now this gets multiplexing past the QPACK barrier so we can see
whether anything else is broken downstream.

https://claude.ai/code/session_01HcvfQq1ttPV9PkRoJb4nyT

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

@@ -27,6 +27,7 @@ import com.vitorpamplona.quic.http3.Http3Frame
 import com.vitorpamplona.quic.http3.Http3FrameReader
 import com.vitorpamplona.quic.http3.Http3FrameType
 import com.vitorpamplona.quic.http3.Http3Settings
+import com.vitorpamplona.quic.http3.Http3SettingsId
 import com.vitorpamplona.quic.http3.Http3StreamType
 import com.vitorpamplona.quic.qpack.QpackDecoder
 import com.vitorpamplona.quic.qpack.QpackEncoder
@@ -62,12 +63,27 @@ class Http3GetClient(
     private val conn: QuicConnection,
 ) : GetClient {
     suspend fun init(scope: CoroutineScope) {
-        // Control stream: type-0x00 prefix followed by a SETTINGS frame
-        // (empty body is legal — RFC 9114 §7.2.4).
+        // Control stream: type-0x00 prefix followed by a SETTINGS frame.
+        // Explicitly advertise QPACK_MAX_TABLE_CAPACITY=0 and
+        // QPACK_BLOCKED_STREAMS=0 — our QpackDecoder is literal-only
+        // (no dynamic-table support, RFC 9204 Required Insert Count = 0).
+        // Empty SETTINGS would let the spec default kick in (also 0),
+        // but aioquic's behavior on multiplexing showed that without
+        // explicit signal it still emits dynamic-table-referenced fields
+        // in response HEADERS, which our decoder silently mis-parses
+        // (status=0, file not written). Sending the explicit zeros
+        // forces aioquic onto the literal-only QPACK path.
         val control = conn.openUniStream()
         val w = QuicWriter()
         w.writeVarint(Http3StreamType.CONTROL)
-        w.writeBytes(Http3Settings(emptyMap()).encodeFrame())
+        w.writeBytes(
+            Http3Settings(
+                mapOf(
+                    Http3SettingsId.QPACK_MAX_TABLE_CAPACITY to 0L,
+                    Http3SettingsId.QPACK_BLOCKED_STREAMS to 0L,
+                ),
+            ).encodeFrame(),
+        )
         control.send.enqueue(w.toByteArray())
         // Control stream stays open for the lifetime of the H3 connection;
         // do NOT call finish() — peers treat that as H3_CLOSED_CRITICAL_STREAM.