Add v0.2.0 test assertions, multi-layer fixture, second atlas

Migrates the test-lib content authored alongside lib-core.maps v0.2.0
into its canonical role=test location. Covers packed-u32 GID encode/
decode round-trip, v1 to v2 schema upgrade, schema-v2 validation,
multi-layer load, multi-atlas resolution, per-layer tile lookup with
tile_at backwards-compat, gameplay queries (is_walkable, blocks_walk,
blocks_sight), layer iteration around the entity slot, and v1 JSON
auto-migration on load. 367 assertions total when run via the engine
test-libs harness.

New fixtures: maps/demo_v2.map.json (4x4 multi-layer with roof) and
assets/tiles/walls_tilemap.tilemap.json (second atlas with wall tiles).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

maps/demo_v2.map.json

@@ -0,0 +1,38 @@
+{
+  "schema_version": 2,
+  "id": "demo_v2",
+  "size": { "w": 4, "h": 4 },
+  "atlases": ["demo_tilemap", "walls_tilemap"],
+  "layers": {
+    "surface": {
+      "tiles": [
+        32, 32, 32, 32,
+        32, 16, 16, 32,
+        32, 16, 16, 32,
+        32, 32, 32, 32
+      ]
+    },
+    "wall": {
+      "tiles": [
+        16777232, 16777232, 16777232, 16777232,
+        16777232, 0, 0, 16777232,
+        16777232, 0, 0, 16777232,
+        16777232, 16777232, 16777232, 16777232
+      ]
+    },
+    "topsurface": {
+      "tiles": [
+        0, 0, 0, 0,
+        0, 0, 16, 0,
+        0, 0, 0, 0,
+        0, 0, 0, 0
+      ]
+    }
+  },
+  "roof": [
+    0, 0, 0, 0,
+    0, 1, 1, 0,
+    0, 1, 1, 0,
+    0, 0, 0, 0
+  ]
+}