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.

Cargo.toml

@@ -0,0 +1,41 @@
+# Cargo.toml — rutster workspace root.
+# Spec ref: slice-1 §2. The workspace pins shared dep versions here so
+# member crates can't drift (§2.1). Each member references with
+# `dep.workspace = true`.
+[workspace]
+resolver = "2"
+members = [
+    "crates/rutster-signaling-sip",
+    "crates/rutster-tap",
+    "crates/rutster-spend",
+]
+
+[workspace.package]
+license = "GPL-3.0-or-later"
+edition = "2021"
+repository = "https://github.com/anomalyco/rutster"
+
+# Pinned versions for all member crates. References are `foo.workspace = true`
+# in the member manifest. Keeps the dep tree unified (§2.1).
+[workspace.dependencies]
+# str0m 0.21: sans-IO WebRTC. Frame API (Event::MediaData + Writer::write).
+str0m = "0.21"
+# opus 0.3.1: libopus FFI (system libopus required — see README).
+opus = "0.3"
+# axum 0.7: HTTP signaling surface.
+axum = { version = "0.7", features = ["macros"] }
+# tokio 1: runtime driving the str0m poll loop (slice-1 deviation per §3.4).
+tokio = { version = "1", features = ["full"] }
+# dashmap 6: in-process session store.
+dashmap = "6"
+# uuid 1: ChannelId newtype backing.
+uuid = { version = "1", features = ["v4"] }
+thiserror = "1"
+tracing = "0.1"
+tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+# tower: used by the binary crate's integration tests (ServiceExt::oneshot
+# 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"] }