Bake stub atlas-sets for demo_tilemap and walls_tilemap

Replaces the legacy color-only *.tilemap.json fixtures with baked
2-tile atlas-sets in the new atlases/<id>/{diffuse,height,atlas-json,
lock-json} layout. Stub PNGs are 32x32 solid colors matching the
previous color values; height channel is synthesized L8=0 by the baker.
A scripts/bake.sh re-bake helper drives the atlas-baker tool against
assets/_sources/.

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

assets/atlases/demo_tilemap/tiles.atlas.json

@@ -0,0 +1,31 @@
+{
+  "atlas_id": "demo_tilemap",
+  "atlas_version": 1,
+  "atlas_size_px": [
+    64,
+    64
+  ],
+  "tile_size_px": 32,
+  "tiles": [
+    {
+      "id": 1,
+      "name": "grass",
+      "uv": [
+        0,
+        0,
+        32,
+        32
+      ]
+    },
+    {
+      "id": 2,
+      "name": "stone",
+      "uv": [
+        32,
+        0,
+        32,
+        32
+      ]
+    }
+  ]
+}

assets/atlases/demo_tilemap/tiles.atlas.lock.json

@@ -0,0 +1,9 @@
+{
+  "atlas_id": "demo_tilemap",
+  "bindings": {
+    "grass": 1,
+    "stone": 2
+  },
+  "deleted": [],
+  "next_id": 3
+}