Split the newest reply once its turn ends, and make the UI harness reusable

The transcript's remaining lag was the newest assistant reply: transcriptUnits
kept the last row whole -- right while it streams (splitting a changing text
is a parse per delta), wrong forever after, so a session that ends on a long
reply drew it as one lazy-list item with every node alive. On a Pixel 9 Pro
XL that was 13.8ms of draw phase a frame, 79% of it the framework's own
per-node bookkeeping, against a 34,996px item.

An AssistantMsg now carries `settled`, folded from the status event that ends
its turn (status changes are transcript events with seqs, so replay settles
the same way), and cleared if a delta ever grows the message again. A settled
newest reply splits like every other. Folding it -- rather than reading the
screen's status -- routes the resplit through the held-events gate, so it can
only happen at the newest end while pinned, never under a reader. The
"session is working" predicate now lives once, in sessionWorking().

Measured on the emulator, same session and gestures, a 43KB reply as the
last row: draw phase 3.92ms -> 1.20ms per frame, framework share 3.07ms
(78%) -> 0.54ms (45%), worst single measure 82.5ms -> 9.1ms. The report's
"on screen" line went from one 60,674px AssistantMsg to five blocks of
95-846px. A live streamed turn settles and splits the moment it goes idle.

The harness half, asked for by Bryan: ui-sandbox.sh now derives its port and
root from the checkout name (two checkouts' sandboxes cannot reach each
other), keeps its token in ~/.config/ai-app/sandbox-token and salvages
enrolled device tokens across restarts (enrol the emulator once, ever), and
gained the driving verbs every UI session was re-inventing in /tmp: spawn,
send (text or @file), api. transcript-bench.sh is the standard
scroll-and-report measurement. AGENTS.md documents all of it.

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

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

@@ -319,7 +319,7 @@ fun SessionScreen(settings: ServerSettings, summary: SessionSummary, onBack: ()
     // them. From the server rather than from this screen, so a rename sent from the settings
     // screen -- or from another device -- is drawn waiting here too.
     var waitingCommands by remember { mutableStateOf(listOf<Pair<String, String>>()) }
-    val running = status == "running" || status == "compacting"
+    val running = sessionWorking(status)
     var moreHistory by remember { mutableStateOf(true) }
     var loadingHistory by remember { mutableStateOf(false) }
     var ready by remember { mutableStateOf(false) }