compaction: live progress signal + user-message cap

COMPACTING event drives a 'Compacting context…' transient in the GUI.
Cap the compacted block's user-message list at 40 with an honest omitted count.

surfaces/gui/src/App.tsx

@@ -172,6 +172,10 @@ export function App() {
   const [mode, setMode] = useState("interactive");
   const [connected, setConnected] = useState(false);
   const [running, setRunning] = useState(false);
+  // Transient "Compacting context…" indicator (OPE-27): set by the `compacting` event,
+  // cleared by whatever the engine emits next — the summarizer call is otherwise a
+  // multi-second silent stall mid-turn.
+  const [compacting, setCompacting] = useState(false);
   const [items, setItems] = useState<Item[]>([]);
   const [streaming, setStreamingState] = useState("");
   // Ref mirror of `streaming`: the WS handler closure is built once per socket and can't read
@@ -576,6 +580,9 @@ export function App() {
           },
         ]);
       };
+      // Any engine event after `compacting` means the summarizer finished (compacted /
+      // silent no-op / failure prompt) — the transient must never outlive it.
+      if (ev.type !== "compacting") setCompacting(false);
       switch (ev.type) {
         case "ready":
           setConnected(true);
@@ -709,6 +716,9 @@ export function App() {
           if (d.model) setModel(d.model);
           setItems((p) => [...p, { kind: "notice", tone: "info", text: d.text || "Model switched" }]);
           break;
+        case "compacting":
+          setCompacting(true);
+          break;
         case "compacted":
           // Auto-compaction marker (OPE-27): outbound-only — the transcript stays intact,
           // this divider just shows where the model's memory was summarized.
@@ -1519,7 +1529,11 @@ export function App() {
                       <ThinkingBlock text={reasoningStream} live />
                     </div>
                   )}
+                  {/* Compaction runs between provider turns (nothing streams during it), so
+                      the transient takes over the waiting slot with a specific label. */}
+                  {running && compacting && <WaitingForAgent label="Compacting context…" />}
                   {running &&
+                    !compacting &&
                     !reasoningStream &&
                     (!streaming || streamMode(streaming, items, running) === "hold") &&
                     !lastItemIsAssistant(items) && <WaitingForAgent />}
@@ -1685,12 +1699,12 @@ function lastItemIsAssistant(items: Item[]): boolean {
   return false;
 }
 
-function WaitingForAgent() {
+function WaitingForAgent({ label }: { label?: string }) {
   return (
     <div className="waiting-transcript">
       <div className="waiting-row" aria-live="polite">
         <span className="waiting-spinner" />
-        <span>Waiting for agent...</span>
+        <span>{label || "Waiting for agent..."}</span>
       </div>
     </div>
   );

surfaces/gui/src/types.ts

@@ -18,6 +18,7 @@ export type EventType =
   | "input_rejected"
   | "interrupted"
   | "model_changed"
+  | "compacting"
   | "compacted"
   | "turn_done";