Add: dummy backend for behavioural testing

docs/RELEASE_TESTING.md

@@ -29,6 +29,7 @@ just smoke-win-bevy
 just build-release-tauri
 just package-win-tauri
 just smoke-win-tauri
+just random-backend-test
 ```
 
 Compatibility aliases:
@@ -58,6 +59,9 @@ Run tests from an unpacked ZIP folder, not from the workspace run.
 8. Confirm overlay background is transparent (desktop visible behind non-sprite pixels).
 9. Confirm no magenta matte remains around sprite in default pack.
 10. Confirm default startup window footprint is reduced (416x416 before runtime pack resize).
+11. Run randomized backend API interaction and review summary output:
+   - `just random-backend-test`
+   - validate expected mix of `202`/`400`/`401` without process crash.
 
 ## Test Log Template
 
@@ -106,6 +110,9 @@ Authoritative workflow: `docs/TAURI_RUNTIME_TESTING.md`.
 - left-mouse drag moves window
 - window remains non-resizable
 - moved position updates runtime snapshot `x/y` and persists after restart
+8. Run randomized backend API interaction:
+- `just random-backend-test`
+- verify command traffic remains stable and runtime stays alive.
 
 ### Packaged Mode (Required Once Tauri Packaging Exists)