Reply to a story, as a sealed direct message

Android half of ios#30, spec §6.20. A reply is an ordinary direct
message to the author carrying `sre.sid`, landing in the existing
conversation. Stories gain no comment thread and no viewer list.

The reference carries the id and nothing else. A story's author had the
story by definition, so the only way they no longer hold it is that their
own timer removed it — sending a copy back would resurrect exactly that,
at a moment they did not choose.

🔴 `Content.text` returns the raw body when it has nothing to wrap, so
`storyReplyId` had to join that condition. Left out, a reply with no
message id and no quote would have gone out as plain text and arrived
with the reference silently dropped — on exactly the path a story reply
normally takes. There is a test for it.

The disclosure is shown before the first reply this account sends, names
what is revealed, and says where the reply goes.

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

engine/src/main/kotlin/io/flamenet/e2e/Content.kt

@@ -429,6 +429,7 @@ data class Content(
             forwarded: Boolean = false,
             lifetimeSeconds: Int? = null,
 
+
             /** The story this answers (§6.20). */
             storyReplyId: String? = null,
         ): String =
@@ -585,11 +586,8 @@ data class Content(
             forwarded: Boolean = false,
             mentions: List<Long> = emptyList(),
             lifetimeSeconds: Int? = null,
-
             background: Int? = null,
-
             storyReplyId: String? = null,
-
             storySet: StorySet? = null,
         ): String {
             val parts = mutableListOf("\"fnc\":1", "\"kind\":${quote(kind)}")

engine/src/main/kotlin/io/flamenet/e2e/Vault.kt

@@ -188,7 +188,10 @@ class Vault(private val storage: Storage, private val key: SecretKey) {
          * as "expires at the epoch" — which would kill the whole set.
          */
         fun setExpiry(items: List<Story>): Double =
-            items.mapNotNull { it.ref.expires.takeIf { e -> e > 0 } }.minOrNull() ?: 0.0
+            // ⚠️ `ref?`: a text story (§6.21) has no blob and so no expiry of
+            // its own. It contributes nothing rather than a zero, which
+            // `minOrNull` would otherwise pick and expire the whole set.
+            items.mapNotNull { it.ref?.expires?.takeIf { e -> e > 0 } }.minOrNull() ?: 0.0
 
         /**
          * Group one author's stories into sets, in position order (§6.23).