Gate library prop collision coverage across venues

MEMORY.md

@@ -4,6 +4,14 @@ Session memory for durable decisions, state, and gotchas. Keep concise, remove o
 
 ## Decisions
 
+- **Collider coverage gate (2026-11-03):** `WorldBuilder` sets `library_prop` metadata on every
+  node in `library_nodes`, and `NON_BLOCKING_LIBRARY` (`marquee`, `wall_lantern`, `handrail`,
+  `double_door`) documents the mounted/decorative pieces that intentionally publish no
+  walk-blocking body. `test_world.gd` verifies placed props have `Collision` + `Shape` +
+  `Avoidance` in all three venues, and that library dressing has a body unless exempt. Both
+  were mutation-checked (removing `_add_prop_collision` fails 3 tests; dropping `handrail` from
+  the exemption list fails with the node name). Suite is 275/275. Add the metadata when adding a
+  new library placement, or the coverage test will report it.
 - **3D review gates (2026-11-03):** `tools/test_blender_assets.py` now gates texel density
   (all five character slots present, 40–400 px/m authored, ≥250 px/m effective fabric), and
   `tools/test_model_review.py` gates scorecard coverage (every `CHARACTERS`/`GEAR`/`PROPS` entry