Tell Compose the transcript's rows are values, so it can leave them alone

A page of history landing recomposed every loaded row and everything
inside it -- 701 compositions for 148 rows in one of Iris's scrolls,
which is four rebuilds of the whole visible transcript, markdown and
all. That is what her `waited` at the 90th percentile was: the frame
could not start because the thread was rebuilding rows whose content had
not changed.

Nothing was stopping Compose skipping them except that it could not
prove it was safe to. Stability is inferred from a class's fields and a
`List` field makes it assume the worst, so `TranscriptItem`,
`TranscriptRow` and `ParsedReplies` were all treated as things that
might change underneath a composable at any moment. They are not: a row
is rebuilt from the transcript rather than edited, two rows describing
the same events are equal, and the parse cache is keyed on the text it
parsed. Saying so is the whole change.

Measured on the emulator over the same scroll: 139 row compositions, and
**15** of them rebuilt the message inside. The other 124 skipped
straight past the markdown, which is where the cost was. The
transcript's own draw came down with it, from 3.1ms mean to 1.2ms.

The promise these annotations make has to stay true -- nothing described
by them is mutated after it is built. It is not today, and the note on
`TranscriptRow` says so where somebody adding a field will read it.

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

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

@@ -3,6 +3,7 @@ package com.example.aiapp
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.Stable
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.ui.Modifier
@@ -166,6 +167,7 @@ private fun parsedMarkdown(text: String, replies: ParsedReplies): State {
  * itself on the way to being finished. It is dropped with the screen, and emptied by the stream
  * reset that drops the rows it describes.
  */
+@Stable
 class ParsedReplies {
     private val parsed = ConcurrentHashMap<String, State>()
 

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

@@ -27,6 +27,7 @@ import androidx.compose.ui.unit.dp
  */
 @Composable
 fun AssistantMessage(text: String, replies: ParsedReplies, modifier: Modifier = Modifier) {
+    DebugStats.count("message composed")
     val parts = remember(text) { partsOf(text) }
     val only = parts.singleOrNull()
     if (only is MessagePart.Prose) {

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

@@ -35,6 +35,7 @@ import androidx.compose.material3.Text
 import androidx.compose.material3.TextButton
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.DisposableEffect
+import androidx.compose.runtime.Immutable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.derivedStateOf
 import androidx.compose.runtime.getValue
@@ -179,6 +180,7 @@ private fun Modifier.holdTopEdge(key: Any, held: TopEdgeHold, hold: (Int) -> Uni
  * stream is the only data source -- opening this screen replays from seq 0, and a reconnect resumes
  * from the last seq seen, so there is no separate history fetch to drift from it.
  */
+@Immutable
 sealed class TranscriptItem {
     /**
      * The transcript sequence number this row started at, and its identity on screen.

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

@@ -18,6 +18,7 @@ import androidx.compose.material3.CircularProgressIndicator
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.Immutable
 import androidx.compose.runtime.remember
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
@@ -38,6 +39,19 @@ import androidx.compose.ui.unit.dp
  * the transcript's own order is what paging and the event stream depend on, and one screen's idea
  * of "these belong together" must not reach back into it.
  */
+/**
+ * Immutable, and said so, because Compose cannot tell.
+ *
+ * A row is a value: it is rebuilt from the transcript rather than edited, and two rows describing
+ * the same events are equal. Compose infers stability from a class's fields, and a `List` field --
+ * which several of these carry -- makes it assume the worst, so every composable taking one
+ * recomposed whenever anything above it did. A page of history landing recomposed all 148 loaded
+ * rows including the markdown inside them, measured as 701 compositions for 148 rows in one scroll,
+ * and that is what a page landing costs on top of the fetch itself.
+ *
+ * The promise this makes is real and has to stay true: nothing here is mutated after it is built.
+ */
+@Immutable
 sealed class TranscriptRow {
     /**
      * This row's identity in the list, which must survive everything that can happen to the row.