feat(render): add procedural texture atlas generation

render/Atlas.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <cstdint>
+#include <vector>
+#include "vf/Block.h"
+
+namespace vf {
+
+// A procedurally generated texture-array atlas: one RGBA tile per layer,
+// indexed by the tile ids that Block.h assigns to each face. Built in code so
+// the demo has no binary asset dependency.
+struct Atlas {
+    int tileSize = 16;
+    int layers = tileCount;
+    std::vector<uint8_t> pixels;   // layers * tileSize * tileSize * 4, RGBA8
+};
+
+Atlas buildAtlas(int tileSize = 16);
+
+} // namespace vf