Add atlas-spec loader with duplicate handling

loadAtlasDirs walks one level deep into each --atlas-dir, picks up
*/tiles.atlas.json, and builds bidirectional name/id maps. On
duplicate atlas_id across paths the first registration wins and a
warning string is returned (callers decide whether to print).

tests/fixtures/atlases-dup/atlas_a/tiles.atlas.json

@@ -0,0 +1,9 @@
+{
+  "atlas_id": "atlas_a",
+  "atlas_version": 1,
+  "atlas_size_px": [32, 32],
+  "tile_size_px": 32,
+  "tiles": [
+    { "id": 1, "name": "ALT_grass", "uv": [0, 0, 32, 32] }
+  ]
+}