feat: module 08 push chains

Movement planned in full, then committed: a Planner builds a plan
(moving and destroyed ids) by running content's movement responses
(OPEN/SHUT, PUSH, STOP) for every step, rolling back failed branches.
Edge of level blocks; YOU objects move furthest-ahead first; rules are
reparsed once after movement. Harness: move/move_dir(Dir), posOf,
kindsAt. Expansion Drop: PULL as one word plus one content response.

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

course/08-push-chains/reference/rust/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "engine"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+# Module 5: cards are JSON data.
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"