feat(rendering): on-screen debug overlay showing current biome and coordinates

Asset-free bitmap-text overlay: a 5x7 font (glyphs defined as verifiable ASCII-art) drawn as solid quads through a tiny 2D shader, white text with a drop shadow for legibility over any background. Renders 'BIOME: <name>' and 'XYZ: x, y, z' top-left, updating live. Adds Shader.SetVector2 and WorldGenerator.BiomeNameAt support; text scale and spawn-height offset read from Configuration.

Assets/shaders/debugtext.frag

@@ -0,0 +1,10 @@
+#version 330 core
+
+out vec4 FragColor;
+
+uniform vec3 u_color;
+
+void main()
+{
+    FragColor = vec4(u_color, 1.0);
+}