feat(app): define trusted rewind branch flow

src/app/turnAdapter.ts

@@ -1,4 +1,11 @@
-import type { PlayerView, PresentationBeat, TurnRequest, TurnResult } from "@contracts";
+import type {
+  ForkBranchRequest,
+  ForkBranchResult,
+  PlayerView,
+  PresentationBeat,
+  TurnRequest,
+  TurnResult
+} from "@contracts";
 
 const DEMO_LATENCY_MS = 120;
 
@@ -95,3 +102,42 @@ export async function submitDemoTurn(
     }
   };
 }
+
+/**
+ * Browser-only rewind preview. The desktop path performs the durable fork in
+ * SQLite; this adapter mirrors the same DTO boundary without claiming local
+ * persistence.
+ */
+export async function forkDemoBranch(
+  request: ForkBranchRequest,
+  currentView: PlayerView
+): Promise<ForkBranchResult> {
+  await new Promise((resolve) => window.setTimeout(resolve, DEMO_LATENCY_MS));
+
+  if (
+    request.storyId !== currentView.storyId ||
+    request.currentBranchId !== currentView.branchId ||
+    request.expectedCurrentNodeId !== currentView.nodeId
+  ) {
+    throw new Error("story branch changed; refresh and retry");
+  }
+
+  const sourceIndex = currentView.history.findIndex((node) => node.id === request.sourceNodeId);
+  if (sourceIndex < 0) throw new Error("selected story node is unavailable");
+
+  const branchId = `branch_${request.actionId.replaceAll(/[^a-zA-Z0-9_.-]/g, "_")}`;
+  const history = currentView.history.slice(0, sourceIndex + 1).map((node) => ({
+    ...node,
+    isCurrent: node.id === request.sourceNodeId
+  }));
+
+  return {
+    branchId,
+    playerView: {
+      ...currentView,
+      nodeId: request.sourceNodeId,
+      branchId,
+      history
+    }
+  };
+}