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-spend/Cargo.toml

@@ -0,0 +1,8 @@
+# crates/rutster-spend/Cargo.toml
+[package]
+name = "rutster-spend"
+version = "0.0.0"
+license.workspace = true
+edition.workspace = true
+repository.workspace = true
+description = "In-boundary spend / abuse gate — stub crate (filled in spearhead step 6)."

crates/rutster-spend/src/lib.rs

@@ -0,0 +1,21 @@
+//! # rutster-spend
+//!
+//! **Status:** stub. Fills in at spearhead step 6 (spend cap / abuse gate).
+//!
+//! In-boundary spend and abuse control is constitutive of the wedge
+//! ([ADR-0002](../../../docs/adr/0002-north-star-and-fused-core.md)): the
+//! runaway brain structurally cannot exceed spend or pacing because it
+//! doesn't hold the wire — the trunk termination + spend gate do, in one
+//! boundary. Pulling spend out into a service re-introduces the 3-vendor
+//! structural hole the fused vertical was chosen to eliminate.
+//!
+//! This crate will hold: spend caps, pacing caps, deny-by-default routing,
+//! rate-limits, toll-fraud pattern detection — co-located with trunk
+//! termination in `rutster-signaling-sip` (step 5). Depends on nothing in
+//! the workspace in slice 1.
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn crate_compiles() {}
+}