feat(vela): retire legacy mocked turn trigger

docs/protocol.md

@@ -15,8 +15,8 @@ Current UI baseline:
 
 - the browser opens a WebSocket directly to `/ws`
 - the UI tracks connection status separately from gateway session status
-- the UI can send `mocked.turn.trigger` after `session.ready` while connected to request one deterministic mocked turn for the active session
 - the UI exposes a push-to-talk mic control shell that sends placeholder `input_audio.append` on press and `input_audio.commit` on release without capturing real audio
+- the push-to-talk shell is the only supported mocked turn entry path from the shipped UI
 
 ## WebSocket Message Envelope
 
@@ -50,7 +50,7 @@ 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
+- `mocked.turn.trigger` is a retired legacy event name that the gateway now rejects with a deterministic recoverable error
 - `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
@@ -59,15 +59,13 @@ 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
+- `mocked.turn.trigger` is rejected deterministically with `error.code = unsupported_mocked_turn_trigger`
 - `input_audio.append` updates the ephemeral session record and moves the session to `listening`
 - each accepted `input_audio.append` emits one deterministic `transcript.partial` for the current placeholder turn
-- `input_audio.commit` emits exactly one deterministic `transcript.final` and then starts the same deterministic mocked assistant response stream used by `mocked.turn.trigger`
-- after a completed placeholder input cycle, the same socket can still send `mocked.turn.trigger`
+- `input_audio.commit` emits exactly one deterministic `transcript.final` and then starts the deterministic mocked assistant response stream for that push-to-talk turn
+- after a completed placeholder input cycle, the same socket can start another placeholder push-to-talk turn without reconnecting
 - `response.cancel` is safe to send even when no mocked turn is active
 - `response.cancel` stops any still-pending mocked turn events for the active turn and 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
@@ -88,7 +86,6 @@ 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 disables the mic control while disconnected, before `session.ready`, or while a mocked turn is already in flight
 - pressing the mic control sends one placeholder `input_audio.append` chunk and releasing it sends `input_audio.commit`
 - while a placeholder push-to-talk turn is in progress, the UI renders the latest `transcript.partial`
@@ -126,26 +123,19 @@ 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
+### Legacy mocked turn trigger rejection
 
-For this increment, `mocked.turn.trigger` produces one fixed interaction for the active session:
+For this increment, direct `mocked.turn.trigger` requests no longer start a mocked turn:
 
 ```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)
+mocked.turn.trigger
+→ error(code="unsupported_mocked_turn_trigger", message="mocked.turn.trigger is no longer supported; use input_audio.append and input_audio.commit instead.")
 ```
 
 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, suppress any later mocked response events for that turn, and return the session to `idle`
+- this rejection is deterministic and recoverable
+- the session remains available for the supported push-to-talk flow on the same socket
 
 ### Deterministic placeholder push-to-talk transcript and mocked response sequence
 
@@ -173,8 +163,8 @@ Safe deterministic edge cases for this mocked placeholder flow:
 
 - commit without any prior append is accepted and emits `transcript.final("[mocked final] Placeholder push-to-talk transcript completed without appended audio.")`
 - repeated appends during one placeholder turn are accepted and each append replaces the latest partial transcript with a chunk-count-based deterministic value
-- after the final transcript, placeholder commit follows the same mocked `thinking → speaking → response.text.delta* → response.completed → idle` path as `mocked.turn.trigger`
-- `response.cancel` can interrupt this mocked post-commit response path the same way it interrupts `mocked.turn.trigger`; already-rendered transcript or assistant text is not retracted
+- after the final transcript, placeholder commit follows the deterministic mocked `thinking → speaking → response.text.delta* → response.completed → idle` path
+- `response.cancel` can interrupt this mocked post-commit response path; already-rendered transcript or assistant text is not retracted
 
 ## Contract Scope for This Increment
 
@@ -207,7 +197,7 @@ Current mocked-pipeline behavior:
 - during an active mocked turn, `response.cancel` returns the session to `idle` immediately
 - any mocked turn timers that have not fired yet are dropped, so no later `response.text.delta` or `response.completed` events are emitted for the cancelled turn
 - the same cancellation behavior applies when a mocked turn was started by `input_audio.commit`
-- once `idle` is restored, the same WebSocket session can start another mocked turn without reconnecting
+- once `idle` is restored, the same WebSocket session can start another placeholder push-to-talk turn without reconnecting
 
 More general future-state expectations: