Off-the-record conversations: the mechanism

A thread held in memory and written to disk by neither side. Close the
app and it is gone: no file to seize, no backup to leak.

It is a proposal, not a switch. The device turning it on sends the
control envelope and keeps writing until the peer echoes it back. A build
that does not know the kind never echoes, so the thread stays on the
record on both sides rather than on one of them, which is worse than not
having the feature because both people would believe the first thing.

Every message reaches disk through one call, so one gate is enough, and a
test pins that count so a second write path cannot quietly walk around
it. Ratchet state is still persisted: messages stop decrypting without
it, and it is not message content.

The control for it comes next.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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

@@ -939,6 +939,23 @@ class Conversations(
         sendPlaintext(toUserId, Content.disappearing(seconds))
     }
 
+    /**
+     * Propose that this conversation goes off the record, or comes back on.
+     *
+     * Nothing is saved here: the proposer waits for the peer to echo it. A
+     * build that does not understand the kind never echoes, so the thread stays
+     * on the record on both sides rather than on one of them.
+     */
+    fun proposeOffTheRecord(
+        toUserId: Long,
+        on: Boolean,
+    ) {
+        sendPlaintext(toUserId, Content.offTheRecord(on))
+    }
+
+    /** Whether this conversation is off the record on this device. */
+    fun offTheRecord(toUserId: Long): Boolean = vault.offTheRecord(Vault.threadForUser(toUserId))
+
     /**
      * Offer a contact the bridge this phone uses (§6.19).
      */
@@ -1455,6 +1472,16 @@ class Conversations(
                 vault.saveDisappearSeconds(Vault.threadForUser(fromUser), seconds)
             }
 
+            Content.Kind.OFF_RECORD -> {
+                val on = Content.offTheRecord(content) ?: return
+                vault.saveOffTheRecord(Vault.threadForUser(fromUser), on)
+                // Answer a proposal so the other side learns this build kept
+                // its side of it. An echo is not echoed back.
+                if (!Content.offTheRecordIsEcho(content)) {
+                    runCatching { sendPlaintext(fromUser, Content.offTheRecord(on, echo = true)) }
+                }
+            }
+
             else -> {
                 Unit
             }