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

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

Assets/shaders/debugtext.vert

@@ -0,0 +1,12 @@
+#version 330 core
+
+// Positions are already in normalized device coordinates; u_offset nudges the
+// whole batch by a few pixels (in NDC) for the drop-shadow pass.
+layout (location = 0) in vec2 aPos;
+
+uniform vec2 u_offset;
+
+void main()
+{
+    gl_Position = vec4(aPos + u_offset, 0.0, 1.0);
+}