feat(ci): verify Expansion Drop solutions; README quick start, disclaimer link; Rust reference lockfiles

- verify-references --expansion overlays expansion/solution on each reference and runs the
  cumulative tests plus the drop test; refuses a solution that touches src/engine/
- CI job verify-expansions
- README: workspace-template + git init step, tool list, repo disclaimer link
- Cargo.lock for every Rust reference (staged with the reference, pins serde for rustc 1.85.1)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

course/02-action-queue/reference/rust/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "engine"
+version = "0.1.0"