feat(slice-3): +rutster-brain-realtime crate skeleton + async-trait dep (spec §1.1)

Workspace member 8 of 8. Library + binary; mirrored slice-2's
rutster-tap-echo shape. Three stub modules (api_key, translator,
openai_client) filled in by Tasks 3-5. No behavioral code yet —
this task only locks the crate boundary + the new async-trait dep
(the Tool trait in Task 6 needs it for async-fns-in-trait-objects).

Cargo.lock

@@ -1182,6 +1182,23 @@ dependencies = [
  "uuid",
 ]
 
+[[package]]
+name = "rutster-brain-realtime"
+version = "0.1.0"
+dependencies = [
+ "async-trait",
+ "base64",
+ "futures-util",
+ "rutster-tap",
+ "serde",
+ "serde_json",
+ "tokio",
+ "tokio-tungstenite",
+ "tracing",
+ "tracing-subscriber",
+ "url",
+]
+
 [[package]]
 name = "rutster-call-model"
 version = "0.0.0"

Cargo.toml

@@ -11,6 +11,7 @@ members = [
     "crates/rutster-trunk",
     "crates/rutster-tap",
     "crates/rutster-tap-echo",
+    "crates/rutster-brain-realtime",
     "crates/rutster-spend",
 ]
 
@@ -51,3 +52,5 @@ futures-util = "0.3"
 url = "2"
 # base64 0.22: PCM <-> base64 codec for the v1 wire format (spec §3).
 base64 = "0.22"
+# async-trait 0.1: async fns in trait objects (Tool trait, slice-3 spec §6.1).
+async-trait = "0.1"

crates/rutster-brain-realtime/Cargo.toml

@@ -0,0 +1,27 @@
+# crates/rutster-brain-realtime/Cargo.toml
+[package]
+name = "rutster-brain-realtime"
+version = "0.1.0"
+license.workspace = true
+edition.workspace = true
+repository.workspace = true
+description = "OpenAI Realtime speech-to-speech brain — translates slice-2's tap protocol to OpenAI Realtime's event schema (green-zone per ADR-0008; slice-3 spec §1.1, §4)."
+
+[dependencies]
+rutster-tap = { path = "../rutster-tap" }
+tokio = { workspace = true, features = ["full"] }
+tokio-tungstenite = { workspace = true, features = ["connect"] }
+futures-util = { workspace = true }
+serde_json = { workspace = true }
+serde = { workspace = true }
+tracing = { workspace = true }
+tracing-subscriber = { workspace = true }
+url = { workspace = true }
+async-trait = { workspace = true }
+base64 = { workspace = true }
+
+[features]
+default = []
+# Mock mode: in-process fake OpenAI Realtime WS server (no real API calls).
+# Used by the integration test + the offline dev loop (spec §7.3).
+mock = []

crates/rutster-brain-realtime/src/api_key.rs

@@ -0,0 +1,2 @@
+//! API-key loader (spec §5.3) — implements the env-var + file-path posture.
+//! Filled in by Task 3.

crates/rutster-brain-realtime/src/lib.rs

@@ -0,0 +1,28 @@
+//! # rutster-brain-realtime
+//!
+//! **Slice-3 brain:** translates slice-2's tap protocol (the green-zone side of
+//! the seam — core-as-client dials this brain's WS server; ADR-0008 classifies
+//! the brain as green-zone) to OpenAI Realtime's event schema. The brain
+//! process is a WS *server* (core-as-client dials it, unchanged from slice-2)
+//! AND a WS *client* to `wss://api.openai.com/v1/realtime` (OpenAI is server
+//! on *its* leg).
+//!
+//! See `docs/superpowers/specs/2026-06-30-slice-3-realtime-brain-design.md` for
+//! the full design.
+//!
+//! ## Modules
+//!
+//! - [`api_key`] — load API key from env var or file (spec §5.3).
+//! - [`translator`] — pure-function event translation (tap ⇄ OpenAI).
+//! - [`openai_client`] — wss:// client to api.openai.com/v1/realtime.
+//!
+//! ## Dev mode (`--features=mock`)
+//!
+//! When built with `mock`, the binary uses in-process `MockRealtimeBrain`
+//! (defined in `lib.rs`'s test-support module) instead of dialing OpenAI.
+//! No API key required, no real OpenAI calls. Used by the integration test +
+//! the offline dev loop (spec §7.3).
+
+pub mod api_key;
+pub mod openai_client;
+pub mod translator;

crates/rutster-brain-realtime/src/openai_client.rs

@@ -0,0 +1 @@
+//! wss://api.openai.com/v1/realtime client (spec §4). Filled in by Task 5.

crates/rutster-brain-realtime/src/translator.rs

@@ -0,0 +1 @@
+//! Tap ⇄ OpenAI Realtime event translation (spec §4). Filled in by Task 4.