feat(vela): add mocked turn transcript response slice

docs/protocol.md

@@ -15,7 +15,7 @@ Current UI baseline:
 
 - the browser opens a WebSocket directly to `/ws`
 - the UI tracks connection status separately from gateway session status
-- the UI currently consumes server events but does not send `session.start` or any audio events yet
+- the UI can send `mocked.turn.trigger` after `session.ready` while connected to request one deterministic mocked turn for the active session
 
 ## WebSocket Message Envelope
 
@@ -40,6 +40,7 @@ This increment intentionally keeps the envelope minimal:
 ```ts
 type ClientEvent =
   | { type: "session.start"; payload: {} }
+  | { type: "mocked.turn.trigger"; payload: {} }
   | { type: "input_audio.append"; payload: { chunk: string } }
   | { type: "input_audio.commit"; payload: {} }
   | { type: "response.cancel"; payload: {} };
@@ -48,6 +49,7 @@ type ClientEvent =
 #### Client event intent
 
 - `session.start` initializes a voice session without locking in transport or auth details yet
+- `mocked.turn.trigger` asks the gateway to run one obviously mocked, deterministic transcript/response turn
 - `input_audio.append` carries a chunk of captured input audio as an encoded string
 - `input_audio.commit` marks the current buffered user turn as ready for downstream processing
 - `response.cancel` interrupts the active listen/think/speak flow
@@ -56,9 +58,12 @@ type ClientEvent =
 
 - on connect, the gateway creates an ephemeral in-memory session and emits `session.ready` plus `session.state`
 - `session.start` is accepted as an idempotent session acknowledgment and re-sends readiness/state
+- `mocked.turn.trigger` is accepted only when no other mocked turn is already in flight for that session
+- a mocked turn emits deterministic `transcript.final`, `response.text.delta`, `response.completed`, and `session.state` events in protocol-valid order
 - `input_audio.append` updates the ephemeral session record and moves the session to `listening`
 - `input_audio.commit` resets the minimal buffered state and returns the session to `idle`
 - `response.cancel` resets the minimal session state back to `idle`
+- a second mocked-turn trigger during an active mocked turn produces `error` with code `mocked_turn_in_flight`
 - malformed JSON produces `error` with code `invalid_json`
 - invalid envelopes or unsupported client event names produce `error` with code `invalid_message`
 - malformed WebSocket frames are rejected without crashing the gateway process
@@ -79,6 +84,7 @@ Notes:
 
 - this UI state is transport-oriented and is separate from the shared gateway `session.state` payload
 - `session.state` currently reflects the gateway session phase (`idle`, `listening`, `thinking`, `speaking`)
+- the UI disables the mocked-turn control until `session.ready` arrives, while disconnected, or while a mocked turn is already in flight
 - the UI treats malformed server messages, browser WebSocket errors, and gateway `error` events as safe error states instead of throwing
 
 ### Server → Client
@@ -109,6 +115,27 @@ type ServerEvent =
 - `response.completed` marks the current assistant turn as done
 - `error` is the minimal recoverable failure shape for both UI and gateway work
 
+### Deterministic mocked turn sequence
+
+For this increment, `mocked.turn.trigger` produces one fixed interaction for the active session:
+
+```text
+session.state(listening)
+→ transcript.final("[mocked user] What is the current mocked vertical slice?")
+→ session.state(thinking)
+→ session.state(speaking)
+→ response.text.delta("[mocked assistant] ")
+→ response.text.delta("This is a deterministic mocked response from the gateway vertical slice.")
+→ response.completed
+→ session.state(idle)
+```
+
+Notes:
+
+- the content is intentionally fixed and obviously mocked
+- no audio, STT, LLM, TTS, or external providers participate in this flow
+- `response.cancel` can stop the mocked turn early and return the session to `idle`
+
 ## Contract Scope for This Increment
 
 This contract is intentionally limited to the smallest event set needed to unblock:
@@ -118,6 +145,7 @@ This contract is intentionally limited to the smallest event set needed to unblo
 
 Explicitly deferred for later increments:
 
+- freeform typed user input
 - tool-calling events
 - streamed TTS/output-audio events
 - reconnect/resume semantics