Migrate test assertions to atlas tile-record shape; bump to 0.3.0

tile.id is now an integer; the stable string identifier is tile.name.
Updates all tile-record string-id assertions to use .name. Adds
walkable field to stub atlas JSONs so gameplay assertions pass without
the legacy tiles dir. Test bootstrap calls maps.load_textures with
aliases pointing to the test-lib itself as the asset provider. Moves
load_textures calls inline after each set_current (Task 4 and Task 9
blocks) so all tile-record shape and walkability checks use atlas
records throughout. Bumps version to 0.3.0, dep on maps 0.3.0.

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

assets/atlases/demo_tilemap/tiles.atlas.json

@@ -10,6 +10,7 @@
     {
       "id": 1,
       "name": "grass",
+      "walkable": true,
       "uv": [
         0,
         0,
@@ -20,6 +21,7 @@
     {
       "id": 2,
       "name": "stone",
+      "walkable": false,
       "uv": [
         32,
         0,