docs: update HANDOFF.md — Stages 10+11 completed, Stage 12 next

src/nsct/api/main.py

@@ -107,6 +107,10 @@ def create_app() -> FastAPI:
     from nsct.api.vision import router as vision_router
     app.include_router(vision_router, tags=["vision"])
 
+    # Mount audio router (Stage 11)
+    from nsct.api.audio import router as audio_router
+    app.include_router(audio_router, tags=["audio"])
+
     return app