docs(slice-3): Python reference brain + LEARNING.md + README dev loop (spec §7.5)

examples/openai_realtime_brain/ -- the canonical foreign-language OpenAI
Realtime brain (Python, ~150 lines, websockets lib only). Not in CI
(zero-non-Rust-dev-deps dev loop per AGENTS.md). Mirrors the slice-2
echo_brain pattern and proves the protocol is language-agnostic.

Hand-rolled against the spec §4.2 mapping rather than using the openai
Python SDK: the SDK is unused by the wire-shape (both sides are plain
WSS), so dropping it removes a dep that the plan skeleton imported but
never called. session.update is sent with turn_detection=null (S4)
matching the Rust brain.

LEARNING.md gains 5 new pointers under a Slice 3 heading: async-trait
in trait objects, the translator pure-function layer, additive protocol
extension via #[serde(other)], the FOB-boundary side-channel mpsc, and
the WS subprotocol handshake (openai_client.rs).

README.md gains the Slice 3 dev loop section (mock mode without an
OpenAI key, real-OpenAI mode, and the Python brain alternative).

.gitignore learns about __pycache__/*.pyc (the Python brains aren't in
CI but byte-compiled artifacts should never be tracked).

LEARNING.md

@@ -79,6 +79,33 @@ can read in `cargo doc --open` plus the source file itself.
   — `TapHandle(())` compiles `Option<TapHandle>` to a single `bool`; no
   runtime cost for the type-system marker.
 
+### Slice 3 — OpenAI Realtime brain
+
+- **`async-trait` patterns / async fns in trait objects** →
+  `crates/rutster/src/tool_registry.rs` — the `Tool` trait's `async fn call`
+  and how `async-trait` lowers it to `Pin<Box<dyn Future + Send>>` (the same
+ boxing the slice-1 `pipe` widening used, now applied to trait methods).
+- **OpenAI Realtime adapter + event translation (pure-function layer)** →
+  `crates/rutster-brain-realtime/src/translator.rs` — the §4.2 event mapping
+  table as pure `fn`s: tap `audio_in` ⇄ OpenAI `input_audio_buffer.append`,
+  `response.audio.delta` ⇄ `audio_out`, `function_call_arguments.done` ⇄
+  `function_call`. Testable without a network (the pump is in a separate file).
+- **Tap protocol additive extension + forward-compat via `#[serde(other)]`** →
+  `crates/rutster-tap/src/protocol.rs` — how slice-3 adds `speech_started`,
+  `speech_stopped`, `function_call`, `function_call_output`, `tools.update`
+  without breaking slice-2 brains: unknown variants deserialize to a catch-all
+  arm instead of erroring (forward-compat by construction).
+- **Side-channel mpsc for FOB-boundary dispatch** →
+  `crates/rutster/src/session_map.rs` — how `drive_all_sessions` drains a
+  function-call mpsc side channel into the tap WS writer without blocking the
+  20 ms media loop. The pattern: hot loop polls `try_recv`; cold path spawns.
+- **HTTP request builder for WS subprotocol handshake** →
+  `crates/rutster-brain-realtime/src/openai_client.rs` — why we hand-roll the
+  WS upgrade `Request` instead of letting `connect_async` build it: to set
+  `Authorization` + `OpenAI-Beta` headers on the WS handshake (tungstenite
+  doesn't expose subprotocol/auth headers via the simple `connect_async(url)`
+  entry point — see `openai_headers` / `openai_realtime_url`).
+
 ## How to read
 
 1. `cargo doc --open` — every module has a `//!` doc comment; the doc