feat(app): define trusted rewind branch flow

crates/nana-domain/src/lib.rs

@@ -720,6 +720,31 @@ pub struct TurnRequest {
     pub input: String,
 }
 
+/// Requests a new playable branch rooted at an already committed story node.
+///
+/// The caller identifies the branch head it observed so the trusted runtime can
+/// reject a rewind racing with a newer turn. The new branch id is generated by
+/// the runtime from these trusted inputs and is never supplied by the UI or the
+/// model.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(rename_all = "camelCase")]
+pub struct ForkBranchRequest {
+    pub story_id: String,
+    pub current_branch_id: String,
+    pub expected_current_node_id: String,
+    pub source_node_id: String,
+    pub action_id: String,
+}
+
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(rename_all = "camelCase")]
+pub struct ForkBranchResult {
+    pub branch_id: String,
+    pub player_view: PlayerView,
+}
+
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(rename_all = "camelCase")]