Add persistent history stacks

docs/circuit-list-editor-api.md

@@ -7,7 +7,14 @@ All paths below are mounted below `/api`. There is no Consumer application API.
 
 Project responses from `GET /projects` and `GET /projects/:projectId` include
 `currentRevision`. It currently exposes the persisted revision foundation only;
-no public history, undo or redo endpoint exists yet.
+no public undo or redo command exists yet.
+
+### Project History State
+
+- `GET /projects/:projectId/history`
+  - returns `currentRevision`, `undoDepth`, `redoDepth` and the current top
+    change-set id for both persistent stacks
+  - is read-only; the editor does not consume it yet
 
 ## Circuit-First Endpoints