feat(vela): mock push-to-talk transcript updates

docs/backlog.md

@@ -38,7 +38,7 @@ Prove the end-to-end interaction model with mocked or stubbed providers.
 - [x] create a minimal UI with mic control
 - [x] create a gateway WebSocket session skeleton
 - [x] implement a mocked transcript/response vertical slice over the existing WebSocket session
-- implement mocked STT flow for partial transcript events
+- [x] 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
 - [x] implement interrupt handling across the mocked pipeline
@@ -190,6 +190,8 @@ Polish the system after the core voice loop is reliable.
 - `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-gateway` now supports placeholder input-audio append/commit cycles before running another mocked turn on the same socket
+- `apps/vela-gateway` now emits deterministic `transcript.partial` events for placeholder `input_audio.append` messages and exactly one deterministic `transcript.final` for each placeholder `input_audio.commit`
+- `apps/vela-ui` now renders the latest placeholder partial transcript during the push-to-talk shell turn and replaces it with the deterministic final transcript on commit
 - `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