The transcript records a message id and a reply reference

🔴 The wire always carried both and Session.Line never stored them.
Content.groupText has taken a messageId since §6.9 shipped and parse
reads it back — the transcript was where it stopped. So nothing here
could name an earlier message: no replies to resolve, no reactions to
fold, no edits to apply. All three were "codec done" and none worked.

A reply from the iPhone app arrived as a bare message with the quote
silently removed, which is worse than the missing id: the reader had no
way to know anything had been dropped.

🔴 Also fixes a real bug in sendGroup. It built the envelope twice —
once to send, once for the outbox — so an id minted inside either would
differ between them, and a member who was offline would eventually
receive a DIFFERENT message from everybody else, which nothing could
reconcile with a reply or a reaction. One id, minted by the caller, used
by both.

Both fields written only when present: a null on every message would
grow the transcript for nothing and make "no id" indistinguishable from
"an id of nothing".

Refs paulhitt/flamenet-messenger-ios#58, #10

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/src/main/kotlin/io/flamenet/client/Conversations.kt

@@ -212,10 +212,22 @@ class Conversations(
      * at the sender as an incoming one. The sender's own copy is written
      * locally by the caller.
      */
-    fun sendToGroup(group: RelayClient.Group, text: String, myUserId: Long): Sent {
+    /**
+     * @param messageId one id for the whole fan-out (§6.9). It is ONE message:
+     *   a reply or a reaction from any member has to name the same thing, so
+     *   per-recipient ids would make that impossible.
+     *
+     * ⚠️ Taken as a parameter rather than minted here, because the caller has
+     * to build the same envelope again for the outbox. Two constructions with
+     * two ids means a queued retry arrives as a *different* message from the
+     * one everybody else got.
+     */
+    fun sendToGroup(group: RelayClient.Group, text: String, myUserId: Long,
+                    messageId: String? = null): Sent {
         val delivered = mutableListOf<String>()
         val failed = LinkedHashMap<String, String>()
-        val body = Content.groupText(text, group.id, groupName = vault.groupName(group.id))
+        val body = Content.groupText(text, group.id, messageId = messageId,
+                                     groupName = vault.groupName(group.id))
 
         for (member in group.memberIds) {
             if (member == myUserId) continue