Draw an opened peer message as list units, and give a peer note its own key

A peer message opened was still one item of the transcript list, so every
block of it was composed, measured, placed and kept *alive* while any part
of it was on screen -- and the framework's own per-frame cost grows with how
many nodes are alive rather than how many are drawn. Measured on the
emulator, scrolling the same stretch with a 43KB message shut, opened, and
opened after this change:

                              shut     opened    opened, split
  draw phase per frame       0.81ms    3.85ms    1.22ms
  of that, the framework     0.39ms    3.15ms    0.42ms
  frame total, median       16.9ms    24.3ms    21.1ms

So an opened message now costs about what a shut one does. That is the
shape the report from the phone had -- 11.80ms of draw phase with 79% of it
outside anything this app times -- which no counter here could attribute,
because "2 units visible" says one of them is enormous without saying which.
It says which now: the transcript section of the report names every visible
unit and its height, which is what found this.

The card is cut up rather than given up. A filled Material card is elevation
zero (`FilledCardTokens.ContainerElevation` is `Level0`), so there is no
shadow for a seam to show through: each piece paints the same fill, rounds
only the corners at the ends of the message, and keeps the 12dp inset the
card's own column had. Opening and shutting still hold the edge the reader
pressed, and now without a correction -- the list is keyed, so it holds the
item it is anchored on wherever the new ones land.

The crash this turned up is the more serious half. `placePeerNote` gives a
note the seq of the turn it started so it sorts above the reply it caused,
and argued the seq was free because it belongs to a status change and a
status draws no row. True, and about the wrong collision: two messages that
arrive during one turn are stamped with the same turn, so they became two
rows with one key and `LazyColumn` threw -- the app dying in the middle of
somebody reading. Two agents writing to a session mid-turn is an ordinary
afternoon. A note now keeps its own arrival seq as its identity while `seq`
stays the position it sorts at, and which value a row is keyed by moved onto
`TranscriptItem` itself, which also removes the `as? ToolRun` branch that
was doing the same job in `TranscriptRow.Single`.

`transcriptUnits` now says which two units collided if it ever happens
again. All the framework's message carries is the key, and when that key is
a seq it names neither row; two lines here answered in one run what had
taken an afternoon.

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

app/androidApp/src/main/kotlin/com/example/aiapp/TranscriptItems.kt

@@ -26,6 +26,18 @@ sealed class TranscriptItem {
      */
     abstract val seq: Long
 
+    /**
+     * This item's identity on screen, which is its [seq] for everything that has one of its own.
+     *
+     * Here rather than in [TranscriptRow.Single] because the two items that need something else are
+     * the two that know why: a tool call is named after its run, and a peer note is *sorted* by the
+     * turn it started rather than by where it arrived. Asking each item what it is called is also
+     * what stops the next such item being missed -- a `when` over concrete types in the row would
+     * have to gain a case, silently, and nothing says when it did not.
+     */
+    open val key: Any
+        get() = seq
+
     data class UserMsg(
         override val seq: Long,
         val text: String,
@@ -72,7 +84,11 @@ sealed class TranscriptItem {
          * breaks -- a screenshot loaded on one page and its call on the next read as unrelated.
          */
         val images: List<String> = emptyList(),
-    ) : TranscriptItem()
+    ) : TranscriptItem() {
+        /** A run, not a seq: see [TranscriptRow.key] for what that identity has to survive. */
+        override val key: Any
+            get() = runId
+    }
 
     data class QuestionCard(
         override val seq: Long,
@@ -97,8 +113,24 @@ sealed class TranscriptItem {
      *
      * Its own row rather than a [UserMsg]: see [PeerMessageRow] for why the voice matters.
      */
-    data class PeerNote(override val seq: Long, val from: String, val text: String) :
-        TranscriptItem()
+    data class PeerNote(
+        override val seq: Long,
+        val from: String,
+        val text: String,
+        /**
+         * The seq of the event this note came in on, which is what makes it itself.
+         *
+         * [seq] is where the note *sorts*, and [placePeerNote] sets it to the seq the turn began at
+         * so the note is drawn above the reply it caused. Two messages that arrive during one turn
+         * therefore share a seq -- and sharing an identity as well killed the app, because the
+         * transcript list refuses two items with one key. Two agents writing to a session mid-turn
+         * is an ordinary afternoon, not a corner.
+         */
+        val arrived: Long = seq,
+    ) : TranscriptItem() {
+        override val key: Any
+            get() = arrived
+    }
 
     /**
      * A command the session ran on itself -- `/compact`, `/rename`.
@@ -278,8 +310,10 @@ private fun adoptRun(
  * where it belongs rather than being drawn out of order at the end.
  *
  * Taking the turn's opening seq as its own is also what keeps the list sorted, which anchors and
- * paging both depend on. That seq belongs to a status change, and a status draws no row, so there
- * is nothing for it to collide with.
+ * paging both depend on. It is only a *position*, though, and the note keeps its own arrival seq as
+ * its identity ([TranscriptItem.PeerNote.arrived]). The argument for sharing was that the turn's
+ * seq belongs to a status change and a status draws no row -- true, and it answered the wrong
+ * question: what two notes stamped with the same turn collide with is each other.
  *
  * Without a stamp -- a message replayed out of a session file, which is already in the right place
  * -- it stays where it arrived.
@@ -290,7 +324,7 @@ private fun placePeerNote(
     event: SessionEvent.PeerMessage,
 ): List<TranscriptItem> {
     val at = event.turnStart ?: return items + TranscriptItem.PeerNote(seq, event.from, event.text)
-    val note = TranscriptItem.PeerNote(at, event.from, event.text)
+    val note = TranscriptItem.PeerNote(at, event.from, event.text, arrived = seq)
     val index = items.indexOfFirst { it.seq > at }
     if (index < 0) return items + note
     val behind = (items.getOrNull(index - 1) as? TranscriptItem.ToolRun)?.runId