feat(vela): support end-to-end mocked turn cancelation

docs/backlog.md

@@ -41,7 +41,7 @@ Prove the end-to-end interaction model with mocked or stubbed providers.
 - implement mocked STT flow for partial transcript events
 - implement mocked LLM response streaming beyond the fixed deterministic slice
 - implement stubbed audio playback or placeholder TTS output
-- implement interrupt handling across the mocked pipeline
+- [x] implement interrupt handling across the mocked pipeline
 
 ### Exit Criteria
 
@@ -188,5 +188,7 @@ Polish the system after the core voice loop is reliable.
 - `apps/vela-gateway` now boots as a minimal Fastify app with `/` and `/health` endpoints
 - `apps/vela-gateway` now exposes a minimal `/ws` WebSocket session skeleton with ephemeral in-memory sessions and defensive message handling
 - `apps/vela-gateway` now accepts `mocked.turn.trigger` and emits protocol-valid mocked transcript/response events with one in-flight mocked turn per session
+- `apps/vela-ui` now exposes a cancel control for active mocked turns and keeps already-rendered transcript/response text visible after cancellation
+- `apps/vela-gateway` now honors `response.cancel` during mocked turns by stopping pending mocked response events, returning the session to `idle`, and allowing a new mocked turn on the same socket
 - `apps/vela-protocol` now provides the shared WebSocket event contract for the UI and gateway
 - backend framework choice is now concrete: Fastify