feat(vela): add mocked turn transcript response slice

docs/architecture.md

@@ -36,13 +36,15 @@ The repository now includes separate runnable workspaces for the UI and gateway
 - PWA enabled
 - WebSocket client
 
-The current implementation is a minimal SvelteKit app with a single voice-session shell page. The shipped UI can open and close a browser WebSocket connection to the gateway `/ws` endpoint, show explicit connection status (`not connected`, `connecting`, `connected`, `disconnected`, `error`), and surface session metadata for developers. Microphone capture, transcript rendering, interrupt controls, streamed assistant response display, and audio playback are not part of the current shell and remain future work.
+The current implementation is a minimal SvelteKit app with a single voice-session shell page. The shipped UI can open and close a browser WebSocket connection to the gateway `/ws` endpoint, show explicit connection status (`not connected`, `connecting`, `connected`, `disconnected`, `error`), trigger one deterministic mocked turn while connected, and render the mocked user transcript plus mocked assistant response for the active session. Microphone capture, real provider integration, and audio playback are still future work.
 
 #### Responsibilities
 
 Current shell responsibilities:
 
 - connection state rendering
+- mocked-turn trigger rendering with disconnected/in-flight guards
+- mocked transcript and mocked assistant response rendering
 - developer-oriented session metadata rendering
 - browser session connect/disconnect controls
 
@@ -60,7 +62,10 @@ Current shell:
 - developer-focused voice-session panel
 - connect button
 - disconnect button
+- mocked-turn button
 - connection status indicator
+- mocked transcript display
+- mocked assistant response display
 - session metadata display
 
 Future interactive voice screen:
@@ -82,7 +87,7 @@ Future interactive voice screen:
 - Fastify (Node)
 - WebSocket-based session layer
 
-The current implementation is a minimal Fastify service with `/`, `/health`, and a documented `/ws` WebSocket session endpoint. The gateway keeps one ephemeral in-memory session record per live socket connection and removes it on disconnect.
+The current implementation is a minimal Fastify service with `/`, `/health`, and a documented `/ws` WebSocket session endpoint. The gateway keeps one ephemeral in-memory session record per live socket connection, removes it on disconnect, and can run one deterministic mocked turn per session without involving any external providers.
 
 #### Responsibilities
 
@@ -100,18 +105,26 @@ The current implementation is a minimal Fastify service with `/`, `/health`, and
 - WebSocket upgrades on `/ws` create an ephemeral session immediately
 - the gateway sends `session.ready` followed by `session.state` (`idle`) when the socket is established
 - valid minimal client events can move the session between `idle` and `listening`
+- `mocked.turn.trigger` drives a fixed transcript/response event sequence over the existing shared protocol
+- only one mocked turn is allowed in flight per session at a time
 - invalid JSON, invalid envelopes, and malformed frames are handled defensively so the process stays up
 
 ### Current UI shell behavior
 
 - renders a minimal developer-focused voice-session panel
-- exposes connect and disconnect controls only
+- exposes connect, disconnect, and mocked-turn controls
 - does not request microphone permission
 - does not send or process audio data
-- reads `session.ready`, `session.state`, and `error` messages from the shared protocol contract
+- reads mocked transcript and mocked response events from the shared protocol contract
 
 ## Voice Pipeline
 
+```text
+Mocked turn button → Gateway mocked session flow → Transcript events → Response text events → UI
+```
+
+This mocked vertical slice intentionally stands in for the future real pipeline:
+
 ```text
 Mic → Gateway → STT → Transcript
 → LLM → Tool Calls → Results