feat(quartz): add group()/groupSet() tag helpers for kind 10009

TagArray.groups()/groupSet() parse the NIP-51 simple-groups list `["group", id,
relay, name?]` items, mirroring ephemChat's rooms()/roomSet(). Needed for the
upcoming relay-group list state (the user's joined groups + servers).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B5MLY4hq5LXJ2D5WeLRyXj

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nip51Lists/simpleGroupList/TagArrayExt.kt

@@ -0,0 +1,29 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.quartz.nip51Lists.simpleGroupList
+
+import com.vitorpamplona.quartz.nip01Core.core.TagArray
+
+/** Parse every `["group", id, relay, name?]` item in this tag array (NIP-51). */
+fun TagArray.groups() = mapNotNull(GroupTag::parse)
+
+/** Parse every `group` item into a de-duplicated set. */
+fun TagArray.groupSet() = mapNotNullTo(mutableSetOf(), GroupTag::parse)