Post a story to a group (§6.24)

Taken by hand, like the two before it — the original was cut from a tree
whose text stories and story replies differ from this branch's.

🔴 The group's roster is the audience, and the story audience does NOT
apply. Those are two different lists making two different statements —
who may see this account's stories, and who is in a conversation — and
they will disagree. The roster wins, because a story addressed to a
group that silently dropped members would be a group message part of the
group cannot see.

`groupStoryReach` returns that overlap rather than a warning. A sender
who believes their story audience still applies is being misled by the
interface, and a disclosure that cannot name the consequence is not one.
The two categories deliberately do not double-count: somebody who is not
a contact was never on that list to be excluded from.

🔴 On receipt, a claimed group id is checked against our own roster. A
story is caught before the group-message branch, so without that check a
sender could attach the id of a group they are not in and have their
story shown as having come from it. The claim is dropped to null rather
than the story refused — the story is legitimately theirs; only the
claim about where it was posted is unverified.

⚠️ `gid` names the audience, never a destination. Rendering a group
story into the group's messages would make a story into a message, and
one that never expires out of it.

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

@@ -345,6 +345,14 @@ class Conversations(
          * the viewer's device contacts that host directly.
          */
         streamUrl: String? = null,
+        /**
+         * The group this story is addressed to (§6.24).
+         *
+         * ⚠️ Names the AUDIENCE, not a destination thread. It rides on the
+         * ordinary story envelope; the recipient decides whether to believe
+         * the claim by checking its own roster.
+         */
+        groupId: String? = null,
     ): StoryPosted {
         val sealed = Attachment.seal(data)
         val id = relay.uploadAttachment(sealed.blob)
@@ -354,7 +362,7 @@ class Conversations(
             duration = duration,
             expires = System.currentTimeMillis() / 1000.0 + lifetimeSeconds,
         )
-        val body = Content.attachment(ref, caption, null, streamUrl = streamUrl)
+        val body = Content.attachment(ref, caption, groupId, streamUrl = streamUrl)
 
         var reached = 0
         var missed = 0