build(slice-2): workspace deps + rutster-tap-echo skeleton

- Add tokio-tungstenite 0.24, futures-util 0.3, url 2, base64 0.22 to
  [workspace.dependencies] (spec §8.1).
- Add crates/rutster-tap-echo as 7th workspace member (spec §2).
- Transition crates/rutster-tap/Cargo.toml from stub to real manifest
  with the deps Tasks 2-4 will need (rutster-media, tokio, tokio-tungstenite,
  serde, serde_json, base64, url, thiserror, tracing).
- Skeleton lib.rs (compile test) + main.rs (placeholder fn main) for
  rutster-tap-echo; Task 5 fills in the echo logic.
- Verify cargo deny check passes against the new transitive dep tree.

Spec ref: docs/superpowers/specs/2026-06-28-slice-2-agent-tap-design.md §2, §8.1.

crates/rutster-tap-echo/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+name = "rutster-tap-echo"
+version = "0.1.0"
+license.workspace = true
+edition.workspace = true
+repository.workspace = true
+
+[dependencies]
+rutster-tap = { path = "../rutster-tap" }       # protocol types — the wire-types-reusable contract test
+tokio = { workspace = true }
+tokio-tungstenite = { workspace = true }
+futures-util = { workspace = true }
+serde_json = { workspace = true }
+tracing = { workspace = true }
+tracing-subscriber = { workspace = true }

crates/rutster-tap-echo/src/lib.rs

@@ -0,0 +1,23 @@
+//! # rutster-tap-echo — the Rust reference echo brain + test server (spec §2.3, §8.4)
+//!
+//! Dual-purpose crate:
+//! - **Standalone binary** (`cargo run -p rutster-tap-echo`): binds
+//!   `ws://127.0.0.1:8081/echo` and echoes `audio_in` → `audio_out` per the
+//!   slice-2 protocol (spec §3). The dev-loop brain the core dials out to.
+//! - **In-process `EchoServer`** (Task 5 lands `EchoServer::start`): used by
+//!   `rutster`'s integration tests to drive the tap end-to-end without an
+//!   external process.
+//!
+//! ## Why a Rust brain at all (when the canonical brain is Python?)
+//!
+//! Reuses `rutster-tap`'s protocol types — **the contract test that the wire
+//! types are reusable from outside the core**. Any future brain written in
+//! Rust (or a step-3 OpenAI adapter in Rust) starts from this shape. The
+//! Python brain (`examples/echo_brain/`) proves language-agnosticism; this
+//! crate proves reusability + powers the in-process integration tests.
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn crate_compiles() {}
+}

crates/rutster-tap-echo/src/main.rs

@@ -0,0 +1,6 @@
+//! Standalone binary: bind `ws://127.0.0.1:8081/echo`, echo audio_in → audio_out.
+//! Real implementation lands in Task 5; this is the skeleton that compiles.
+
+fn main() {
+    eprintln!("rutster-tap-echo: skeleton — implementation lands in Task 5");
+}

crates/rutster-tap/Cargo.toml

@@ -1,8 +1,18 @@
-# crates/rutster-tap/Cargo.toml
 [package]
 name = "rutster-tap"
-version = "0.0.0"
+version = "0.1.0"
 license.workspace = true
 edition.workspace = true
 repository.workspace = true
-description = "Agent audio tap — stub crate (filled in spearhead step 2)."
+
+[dependencies]
+rutster-media = { path = "../rutster-media" }   # PcmFrame (re-exported — spec §3.1)
+tokio = { workspace = true }
+tokio-tungstenite = { workspace = true }
+futures-util = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+base64 = { workspace = true }
+url = { workspace = true }
+thiserror = { workspace = true }
+tracing = { workspace = true }