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.

deny.toml

@@ -111,6 +111,24 @@ skip = [
     # together. Both pull distinct `r-efi` versions, so the two skips
     # are coupled: skip one without the other and the dup remains.
     "getrandom@0.3.4",
+    # `rand` / `rand_core` / `rand_chacha` / `getrandom` v0.2-era split
+    # (slice-2 §8.1 task 1): `tokio-tungstenite` = "0.24" → `tungstenite`
+    # 0.24 pulls the OLD `rand` 0.8 chain (rand_core 0.6, rand_chacha 0.3,
+    # getrandom 0.2), while `str0m` (via `dimpl`/`sctp-proto`) and the
+    # uuid branch use the NEW `rand` 0.9 chain (rand_core 0.9,
+    # rand_chacha 0.9, getrandom 0.4). Both branches are fully transitive
+    # (we don't depend on `rand` directly). Unified only by either
+    # bumping `tokio-tungstenite` to a release that adopts `rand` 0.9
+    # (none of the 0.24.x line does; newer 0.25+ requires features beyond
+    # slice-2's verbatim dep pin per spec §8.1), or forking tungstenite —
+    # both out of scope for Task 1. These four skips are coupled: skip
+    # rand@0.8.6 without its three peers and the duplicates remain.
+    # Revisit when `tokio-tungstenite` workspace dep moves to a release
+    # that unifies on `rand` 0.9.
+    "rand@0.8.6",
+    "rand_core@0.6.4",
+    "rand_chacha@0.3.1",
+    "getrandom@0.2.17",
 ]
 skip-tree = []