feat(vagrant): spawn rock entity with pickup trigger

- Rock template (stack_mode=individual, tags=renderable+item) defined
  alongside backpack and vagrant_player templates.
- state.rock entity created in M.init using slot_00_isolated UV from
  the blob_rect_stone atlas (lib-asset.prototype-blob-geom); position
  (270, 400) — 80px left of the backpack.
- register_pickup(state.rock) wires an interaction trigger; existing
  render.draw_entities{tag="renderable"} draws the rock with no
  special-case render code.
- CI trace: engine.print("vagrant: rock_spawned") emitted once on init.
- Version bump 0.7.2 -> 0.7.3.

manifest.module

@@ -1,6 +1,6 @@
 {
     "id": "vagrant-skeleton",
-    "version": "0.7.2",
+    "version": "0.7.3",
     "kind": "module",
     "api": "^0.1",
     "ci_frames": 60,