workspace: scaffold + three stub crates (sip/tap/spend)

Workspace root, pinned toolchain, and the three stub crates whose only
job in slice 1 is to lock the ADR-0002 boundary shape. Each ships a
lib.rs module doc (what it will hold, why deferred, which spearhead step
fills it) and a crate_compiles test. Spec §2.2.

crates/rutster-tap/src/lib.rs

@@ -0,0 +1,21 @@
+//! # rutster-tap
+//!
+//! **Status:** stub. Fills in at spearhead step 2 (the tap itself).
+//!
+//! Slice 1 *pre-paves* the tap by exposing the canonical PCM boundary as
+//! the `AudioSource` / `AudioSink` traits in [`rutster_media`](../rutster-media/index.html),
+//! and wires an `EchoAudioPipe` between sink and source. Step 2 swaps that
+//! pipe for a real WSS tap client (core-as-client, brain-as-server —
+//! [ADR-0006](../../../docs/adr/0006-ingress-posture.md)). No code changes to
+//! `RtcSession` itself in step 2 — that's the test of the seam.
+//!
+//! This crate will, when filled in, re-export `PcmFrame` from
+//! `rutster-media` (one canonical home — spec §3.1) and ship the WSS
+//! tap client + the versioned framing protocol. It depends on nothing in
+//! the workspace in slice 1.
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn crate_compiles() {}
+}