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/_sources/_gen.js

@@ -0,0 +1,21 @@
+const { PNG } = require('pngjs');
+const fs = require('node:fs');
+const path = require('node:path');
+
+function writeSolidRGBA(p, w, h, rgba) {
+    const png = new PNG({ width: w, height: h });
+    for (let i = 0; i < png.data.length; i += 4) {
+        png.data[i + 0] = rgba[0];
+        png.data[i + 1] = rgba[1];
+        png.data[i + 2] = rgba[2];
+        png.data[i + 3] = rgba[3];
+    }
+    fs.writeFileSync(p, PNG.sync.write(png));
+}
+
+const SOURCES = path.resolve(__dirname);
+writeSolidRGBA(path.join(SOURCES, 'demo_tilemap',  'grass_diffuse.png'), 32, 32, [110, 170,  80, 255]);
+writeSolidRGBA(path.join(SOURCES, 'demo_tilemap',  'stone_diffuse.png'), 32, 32, [120, 120, 120, 255]);
+writeSolidRGBA(path.join(SOURCES, 'walls_tilemap', 'wall_brick_diffuse.png'), 32, 32, [180, 100,  80, 255]);
+writeSolidRGBA(path.join(SOURCES, 'walls_tilemap', 'wall_wood_diffuse.png'),  32, 32, [140,  90,  50, 255]);
+console.log('test-stub sources generated');