slice-3 §5.3 dev mode: an in-process fake OpenAI Realtime WS server
(MockRealtimeBrain) behind --features=mock. Asserts session.update has
turn_detection: null (S4 — spec §4.3) on handshake; exports a typed
OpenAI-shaped error on violation so the brain fails fast + visibly.
On each input_audio_buffer.append replies with a canned
response.audio.delta carrying 480 zeroed samples as base64 LE i16
(same wire shape as slice-2's audio_out). Powers the offline dev loop
(cargo run -p rutster-brain-realtime --features=mock) + the integration
test (no real OpenAI credentials, no network calls).
Also adds the binary main.rs (spec §4.2 + §5.3): binds the tap-side WS
server on RUTSTER_TAP_BIND (default 127.0.0.1:8082), accepts per-call
tap connections, handles the hello handshake, splits the WS into sink +
stream, dials the OpenAI side (real wss://api.openai.com in default
mode; MockRealtimeBrain url in mock mode), and bridges via
run_openai_pump + two mpsc::channel<String> pairs.
Integration test brain_mock_round_trip drives MockRealtimeBrain +
run_openai_pump end-to-end: pushes a tap audio_in frame through the
pump, observes the canned response.audio.delta arrive + get translated
back as a tap audio_out frame on the pump's outbound mpsc. Verifies
the dev-loop 'actually start + interop on a basic audio round-trip'
contract the directive's Task 4 calls out, without requiring a real
WebRTC peer (same constraint as slice-1/2's browser-driven e2e).
Files: crates/rutster-brain-realtime/src/{lib,mock,main}.rs,
crates/rutster-brain-realtime/tests/brain_mock_round_trip.rs.
NOT touched: loop_driver.rs, rtc_session.rs (seam §7.5 #6).
Gates: cargo fmt OK. cargo clippy --all --tests --features=mock
-D warnings OK. cargo test --all --features=mock OK (56 tests).
cargo deny check has pre-existing environmental failure (CVSS 4.0
unsupported in advisory-db; same on main).
Dev loop verified: cargo run -p rutster-brain-realtime --features=mock
starts the brain (mock on ephemeral port, tap server on :8082).
./target/release/rutster with RUTSTER_TAP_URL=ws://127.0.0.1:8082/realtime
starts the core on :8080. POST /v1/sessions succeeds. (Full audio
round-trip requires a browser-driven WebRTC peer, same constraint as
slice-1/2 — covered by the integration test instead.)
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