⏺ fix: derive nostrGroupId from MLS GroupContext in Welcome processing

The "h" tag is optional in MIP-02 Welcome events — senders like
  whitenoise-rs omit it. Instead of failing when the h-tag is absent,
  derive nostrGroupId from the NostrGroupData extension embedded in the
  Welcome's GroupContext (the authoritative MLS source). An h-tag hint,
  if present, is still validated against the MLS-derived value.

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/marmot/MarmotInboundProcessor.kt

@@ -219,26 +219,21 @@ class MarmotInboundProcessor(
      * 4. Mark KeyPackage as consumed for rotation
      *
      * @param welcomeEvent the unwrapped kind:444 event
-     * @param nostrGroupId the Nostr group ID (from relay context or Welcome tags)
+     * @param hintNostrGroupId optional group ID from the "h" tag; validated against MLS content
+     *   if provided. If absent (sender omitted "h" tag), the ID is derived from the Welcome's
+     *   NostrGroupData extension — the MLS content is always the authoritative source.
      * @return the processing result
      */
     @OptIn(ExperimentalEncodingApi::class)
     suspend fun processWelcome(
         welcomeEvent: WelcomeEvent,
-        nostrGroupId: HexKey,
+        hintNostrGroupId: HexKey? = null,
     ): WelcomeResult =
         try {
             com.vitorpamplona.quartz.utils.Log
                 .d("MarmotDbg") {
-                    "MarmotInboundProcessor.processWelcome: group=${nostrGroupId.take(8)}… eventId=${welcomeEvent.id.take(8)}…"
+                    "MarmotInboundProcessor.processWelcome: hint=${hintNostrGroupId?.take(8)} eventId=${welcomeEvent.id.take(8)}…"
                 }
-            // Validate the caller-provided nostrGroupId matches the Welcome event's own h tag
-            val eventGroupId = welcomeEvent.nostrGroupId()
-            if (eventGroupId != null && eventGroupId != nostrGroupId) {
-                return WelcomeResult.Error(
-                    "nostrGroupId mismatch: caller=$nostrGroupId, event=$eventGroupId",
-                )
-            }
 
             val welcomeBytes = Base64.decode(welcomeEvent.welcomeBase64())
             val keyPackageEventId = welcomeEvent.keyPackageEventId()
@@ -273,10 +268,11 @@ class MarmotInboundProcessor(
             com.vitorpamplona.quartz.utils.Log
                 .d("MarmotDbg") { "MarmotInboundProcessor.processWelcome: bundle found — invoking groupManager.processWelcome" }
 
-            // Join the group
-            groupManager.processWelcome(nostrGroupId, welcomeBytes, bundle)
+            // Join the group; nostrGroupId is derived from the MLS GroupContext's
+            // NostrGroupData extension. The h-tag hint (if any) is validated inside.
+            val (_, nostrGroupId) = groupManager.processWelcome(welcomeBytes, bundle, hintNostrGroupId)
             com.vitorpamplona.quartz.utils.Log
-                .d("MarmotDbg") { "MarmotInboundProcessor.processWelcome: groupManager.processWelcome succeeded for ${nostrGroupId.take(8)}…" }
+                .d("MarmotDbg") { "MarmotInboundProcessor.processWelcome: joined group=${nostrGroupId.take(8)}…" }
 
             // Mark the KeyPackage as consumed — triggers rotation
             keyPackageRotationManager.markConsumedByEventId(keyPackageEventId)

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/marmot/mls/group/MlsGroupManager.kt

@@ -192,25 +192,40 @@ class MlsGroupManager(
      * 2. Group state is persisted
      * 3. A KeyPackage rotation should be triggered (see [needsKeyPackageRotation])
      *
-     * @param nostrGroupId hex-encoded Nostr group ID (from Welcome event tags)
      * @param welcomeBytes TLS-serialized Welcome message
      * @param bundle the KeyPackageBundle that was used for the invitation
-     * @return the joined [MlsGroup]
+     * @param hintNostrGroupId optional nostrGroupId from the Welcome event's "h" tag;
+     *   if provided and non-null, validated against the GroupContext's NostrGroupData extension.
+     *   If absent (sender did not include an "h" tag), the ID is derived from the MLS content.
+     * @return pair of (joined group, derived nostrGroupId)
      */
     suspend fun processWelcome(
-        nostrGroupId: HexKey,
         welcomeBytes: ByteArray,
         bundle: KeyPackageBundle,
-    ): MlsGroup =
+        hintNostrGroupId: HexKey? = null,
+    ): Pair<MlsGroup, HexKey> =
         mutex.withLock {
             val group = MlsGroup.processWelcome(welcomeBytes, bundle)
-            groups[nostrGroupId] = group
+
+            val derivedId =
+                group.currentMarmotData()?.nostrGroupId
+                    ?: throw IllegalArgumentException(
+                        "Welcome GroupContext is missing the NostrGroupData extension — cannot derive nostrGroupId",
+                    )
+
+            if (hintNostrGroupId != null && hintNostrGroupId != derivedId) {
+                throw IllegalArgumentException(
+                    "nostrGroupId mismatch: h-tag=$hintNostrGroupId, GroupContext=$derivedId",
+                )
+            }
+
+            groups[derivedId] = group
 
             // init_key is consumed — the bundle's initPrivateKey should not be
             // reused. Caller must discard the bundle and rotate KeyPackages.
 
-            persistGroup(nostrGroupId)
-            group
+            persistGroup(derivedId)
+            Pair(group, derivedId)
         }
 
     /**