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.

rust-toolchain.toml

@@ -0,0 +1,4 @@
+# rust-toolchain.toml — pins the toolchain for reproducible builds.
+[toolchain]
+channel = "1.80"
+components = ["rustfmt", "clippy"]