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.

Cargo.toml

@@ -10,6 +10,7 @@ members = [
     "crates/rutster-media",
     "crates/rutster-signaling-sip",
     "crates/rutster-tap",
+    "crates/rutster-tap-echo",
     "crates/rutster-spend",
 ]
 
@@ -42,3 +43,11 @@ serde_json = "1"
 # on the axum Router). Axum re-exports parts of tower but the integration test
 # uses `tower::ServiceExt` directly, so it needs to be a workspace dep.
 tower = { version = "0.5", features = ["util"] }
+# tokio-tungstenite 0.24: WS client + server (slice-2 tap transport).
+tokio-tungstenite = { version = "0.24", features = ["connect"] }
+# futures-util 0.3: Sink/Stream traits for WebSocketStream.
+futures-util = "0.3"
+# url 2: tap URL parsing + host validation (spec §4.4).
+url = "2"
+# base64 0.22: PCM <-> base64 codec for the v1 wire format (spec §3).
+base64 = "0.22"