fix: recover session submissions and failed turn settlement

app/api/chats/[id]/route.ts

@@ -1,4 +1,4 @@
-import { compactionInFlight, deleteChatSession, getChat, isTurnRunning, sessionCoverage, sessionCost, stores, type ChatDeleteMode } from "@/src/server";
+import { compactionInFlight, deleteChatSession, getChat, isTurnRunning, sessionCoverage, sessionCost, stores, turnFailure, type ChatDeleteMode } from "@/src/server";
 
 export async function GET(_req: Request, { params }: { params: Promise<{ id: string }> }) {
 	const { id } = await params;
@@ -19,6 +19,8 @@ export async function GET(_req: Request, { params }: { params: Promise<{ id: str
 		messages: chat.messages,
 		pricing: sessionCost(chat),
 		turnInFlight: isTurnRunning(id),
+		// Also reaches idle views that missed the failing turn's stream.
+		turnError: turnFailure(id),
 		compacting: compactionInFlight(id),
 		coverage: sessionCoverage(id),
 	});

app/api/chats/[id]/stream/route.ts

@@ -5,8 +5,9 @@
  * submitter too attaches here. One SSE stream per chat: a running turn →
  * subscribe to the live turn object: FULL replay of the event log, then
  * live chunks until settle ([DONE]). No running turn → the cheap terminal
- * ({"type":"turn-settled"} + [DONE]): the record is current, the client
- * refetches it.
+ * ({"type":"turn-settled"} + [DONE]): the client refetches the record.
+ * A retained settlement-failure notice precedes that terminal when a
+ * reply could not be saved before the submitter attached.
  *
  * 404 mirrors the POST handler's unknown-chat. Framework route is a thin
  * wrapper over the server barrel (the import law); tests drive the seams
@@ -18,6 +19,6 @@ export async function GET(_req: Request, ctx: { params: Promise<{ id: string }>
 	const { id } = await ctx.params;
 	if (!getChat(id)) return Response.json({ error: "Unknown chat" }, { status: 404 });
 	const handle = runningTurn(id);
-	if (!handle) return settledTurnResponse();
+	if (!handle) return settledTurnResponse(id);
 	return piTurnResponse(handle);
 }

app/chat/[id]/page.tsx

@@ -1,7 +1,7 @@
 import { notFound } from "next/navigation";
 import { Chat } from "@/components/chat";
 import { slotsConfigured } from "@/src/core";
-import { compactionInFlight, getChat, isTurnRunning, readSettingsRecord } from "@/src/server";
+import { compactionInFlight, getChat, isTurnRunning, readSettingsRecord, turnFailure } from "@/src/server";
 
 export const dynamic = "force-dynamic";
 
@@ -29,6 +29,7 @@ export default async function ChatPage({
 				// derived app-side (core/view.ts projectView; GET serves the same
 				// native messages).
 				initialMessages={chat.messages}
+			initialError={turnFailure(chat.id)}
 			initialConfigured={slotsConfigured(record?.models)}
 			initialPrompt={prompt}
 			// A teacher turn that outlived its client: the remounted view must