fix(nests): reply SubscribeDrop on unsupported tracks instead of silent FIN

When a relay opens a SUBSCRIBE bidi for a track this session doesn't
publish (e.g. a watcher subscribes to `video/data` against a broadcast
that only declares `audio/data` + `catalog.json`), the session was
silently FINing the bidi without writing any reply. The peer's wait
on its receive side resolves to end-of-stream with no indication WHY —
indistinguishable from "publisher disappeared mid-subscribe."

Reply with [MoqLiteSubscribeDrop] carrying
[MoqLiteSubscribeDropCode.TRACK_DOES_NOT_EXIST] (0x04, mirroring the
IETF-MoQ `ErrorCode.TRACK_DOES_NOT_EXIST` convention) and an
informational reason phrase listing the tracks we DO publish, then
FIN. Matches what kixelated's `rs/moq-lite/src/lite/subscribe.rs`
expects on this path.

Doesn't change the happy path — every track NestsUI-v2 subscribes to
(`audio/data` + `catalog.json`) is now published — but is the
spec-correct behaviour for any future watcher that prospectively
subscribes to a track our publisher hasn't declared.

https://claude.ai/code/session_014JfZJHSTvyYYWJbC9VbB47

nestsClient/src/commonMain/kotlin/com/vitorpamplona/nestsclient/moq/lite/MoqLiteMessages.kt

@@ -231,14 +231,40 @@ data class MoqLiteSubscribeOk(
  * RESET_STREAM, but the publisher can pre-emptively decline a
  * subscription with [MoqLiteSubscribeDrop] before any group flows.
  *
- * Decode-only as far as the client cares — we never send Drop back
+ * Sent by [MoqLiteSession] when a relay opens a SUBSCRIBE bidi for a
- * upstream.
+ * `track` we don't publish (e.g. a watcher subscribes to a `video/data`
+ * rendition but our broadcast only declares `audio/data` + `catalog.json`
+ * in its catalog). Without a Drop reply the watcher's response wait
+ * resolves only when the bidi is FIN'd, with no indication WHY — Drop
+ * carries the error code and a reason phrase the watcher can log /
+ * surface.
+ *
+ * Decoded by [MoqLiteSession] when the relay or upstream publisher
+ * rejects one of OUR subscriptions — we surface it as
+ * [MoqLiteSubscribeException] so callers see a typed protocol-level
+ * rejection rather than a silent end-of-flow.
  */
 data class MoqLiteSubscribeDrop(
     val errorCode: Long,
     val reasonPhrase: String,
 )
 
+/**
+ * Error codes carried in the [MoqLiteSubscribeDrop.errorCode] varint.
+ * moq-lite leaves these application-defined; we mirror the IETF-MoQ
+ * conventions in `com.vitorpamplona.nestsclient.moq.ErrorCode` so a
+ * cross-protocol reader gets the same semantic from either path.
+ */
+object MoqLiteSubscribeDropCode {
+    /**
+     * The publisher does not serve this `(broadcast, track)` tuple.
+     * Sent for a subscribe whose `track` doesn't match any of the
+     * publishers we registered on this session. Mirrors
+     * `com.vitorpamplona.nestsclient.moq.ErrorCode.TRACK_DOES_NOT_EXIST`.
+     */
+    const val TRACK_DOES_NOT_EXIST: Long = 0x04L
+}
+
 /**
  * Header at the start of a Group uni stream. After the
  * [MoqLiteDataType.Group] type byte, the publisher writes one

nestsClient/src/commonMain/kotlin/com/vitorpamplona/nestsclient/moq/lite/MoqLiteSession.kt

@@ -849,11 +849,34 @@ class MoqLiteSession internal constructor(
                             // single-track-per-publisher model, only one match is possible.
                             val targetPublisher = publishersSnapshot.firstOrNull { it.track == sub.track }
                             if (targetPublisher == null) {
+                                // Reply SubscribeDrop with a TRACK_DOES_NOT_EXIST
+                                // error code BEFORE we FIN — without this the
+                                // peer's response wait resolves only on
+                                // bidi-FIN with no indication WHY (looks
+                                // identical to "publisher disappeared mid-
+                                // subscribe"). Drop carries the error code +
+                                // reason phrase the watcher can log /
+                                // surface, and matches what kixelated's
+                                // `rs/moq-lite/src/lite/subscribe.rs`
+                                // expects for an unrecognised track on a
+                                // live broadcast.
                                 Log.w("NestTx") {
                                     "SUBSCRIBE inbound id=${sub.id} track='${sub.track}' has no matching publisher " +
-                                        "on this session (have ${publishersSnapshot.map { it.track }}) — finishing bidi"
+                                        "on this session (have ${publishersSnapshot.map { it.track }}) — replying SubscribeDrop"
+                                }
+                                runCatching {
+                                    bidi.write(
+                                        MoqLiteCodec.encodeSubscribeDrop(
+                                            MoqLiteSubscribeDrop(
+                                                errorCode = MoqLiteSubscribeDropCode.TRACK_DOES_NOT_EXIST,
+                                                reasonPhrase =
+                                                    "track '${sub.track}' is not published on this broadcast " +
+                                                        "(available: ${publishersSnapshot.joinToString(",") { it.track }})",
+                                            ),
+                                        ),
+                                    )
+                                    bidi.finish()
                                 }
-                                runCatching { bidi.finish() }
                                 dispatched = true
                                 return@collect
                             }

nestsClient/src/commonTest/kotlin/com/vitorpamplona/nestsclient/moq/lite/MoqLiteSessionTest.kt

@@ -357,6 +357,50 @@ class MoqLiteSessionTest {
             session.close()
         }
 
+    @Test
+    fun publisher_replies_subscribeDrop_when_track_is_not_published() =
+        runBlocking {
+            val (clientSide, serverSide) = FakeWebTransport.pair()
+            val session = MoqLiteSession.client(clientSide, pumpScope)
+
+            // Publisher serves audio/data only.
+            val publisher = session.publish(broadcastSuffix = "speakerPubkey", track = "audio/data")
+
+            // Relay opens a Subscribe bidi for a DIFFERENT track. The
+            // session must reply with SubscribeDrop carrying the
+            // TRACK_DOES_NOT_EXIST code rather than a silent FIN —
+            // otherwise the watcher's response wait resolves only when
+            // the bidi is FIN'd, with no indication WHY (looks
+            // identical to "publisher disappeared mid-subscribe").
+            val subBidi = serverSide.openBidiStream()
+            subBidi.write(Varint.encode(MoqLiteControlType.Subscribe.code))
+            subBidi.write(
+                MoqLiteCodec.encodeSubscribe(
+                    MoqLiteSubscribe(
+                        id = 99L,
+                        broadcast = "speakerPubkey",
+                        track = "video/data",
+                        priority = 0x80,
+                        ordered = true,
+                        maxLatencyMillis = 0L,
+                        startGroup = null,
+                        endGroup = null,
+                    ),
+                ),
+            )
+
+            val ackChunk = withTimeout(2_000) { subBidi.incoming().first() }
+            val resp = MoqLiteCodec.decodeSubscribeResponse(ackChunk)
+            val dropped = resp as MoqLiteCodec.SubscribeResponse.Dropped
+            assertEquals(MoqLiteSubscribeDropCode.TRACK_DOES_NOT_EXIST, dropped.drop.errorCode)
+            // Reason phrase is informational; pin substring rather than
+            // the exact text so we can keep tweaking the wording.
+            kotlin.test.assertContains(dropped.drop.reasonPhrase, "video/data")
+
+            publisher.close()
+            session.close()
+        }
+
     @Test
     fun publisher_close_emits_ended_announce() =
         runBlocking {