Tell somebody when their disappearing message is screenshotted

Android half of ios#36, spec §6.27. `shot` joins the control kinds, so a
build that does not know it stays quiet rather than rendering JSON.

🔴 The care is in refusing to let the absence of a notice mean anything.
The device that took the screenshot reports it, so a modified build, a
second phone, or a screen recorder produce nothing. A notice means one
was probably taken; no notice means NOTHING, and nothing counts them.

⚠️ Android's half of the asymmetry is stated rather than hidden. There is
no screenshot callback at all before API 34, and the one added there does
not cover screen recording — so the disclaimer says that, in those words.
Claiming a symmetry with iOS that does not exist would be worse than the
gap, because somebody would rely on it.

⚠️ Only where a timer is set, and the timer check comes before the
rate-limit check so a suppression window cannot make an untimed
conversation start reporting.

⚠️ Last-notice times are in memory. Persisting them would store who
screenshotted whom on disk to enforce a five-second window.

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

app/src/main/kotlin/io/flamenet/messenger/app/Session.kt

@@ -1222,6 +1222,30 @@ class Session(
         return posted
     }
 
+    /**
+     * When a screenshot notice was last sent to each peer (§6.27).
+     *
+     * ⚠️ In memory only. The window it enforces is seconds long, so persisting
+     * it would be storing who screenshotted whom — a record of exactly the
+     * thing this feature exists to report — on disk, for no benefit.
+     */
+    private val lastScreenshotNotice = mutableMapOf<Long, Long>()
+
+    /**
+     * Tell this peer their disappearing message was screenshotted (§6.27).
+     *
+     * ⚠️ Best effort and deliberately silent on failure. A notice that could
+     * not be sent is not something to surface to the person who took the
+     * screenshot — they did not ask for it and cannot act on it.
+     */
+    fun announceScreenshot(peerUserId: Long, nowMillis: Long = System.currentTimeMillis()) {
+        val timer = vault.disappearSeconds(Vault.threadForUser(peerUserId))
+        if (!Content.shouldAnnounceScreenshot(
+                timer, lastScreenshotNotice[peerUserId], nowMillis)) return
+        lastScreenshotNotice[peerUserId] = nowMillis
+        runCatching { conversations.send(peerUserId, Content.screenshot(nowMillis / 1000)) }
+    }
+
     /**
      * Members of a group we are in, or null if the relay does not list it for
      * us (audit H1). Cached for a short window: a poll pass can carry many