Add shadow frustum culling to terrain and M2 depth passes

Both passes were rendering the entire loaded scene (17×17 tile radius)
into a shadow map that only covers 360×360 world units — submitting
10-50× more geometry than the shadow frustum can actually use.

- TerrainRenderer::renderShadow: skip chunks whose bounding sphere
  doesn't overlap the shadow frustum AABB in XY. Reduces terrain draw
  calls from O(all loaded chunks) to O(chunks within ~180 units).
- M2Renderer::renderShadow: skip instances whose world AABB doesn't
  overlap the shadow frustum in XY. Reduces M2 draw calls similarly.
- Both functions now take shadowCenter + halfExtent parameters.

include/rendering/m2_renderer.hpp

@@ -236,7 +236,7 @@ public:
     /**
      * Render depth-only pass for shadow casting
      */
-    void renderShadow(GLuint shadowShaderProgram);
+    void renderShadow(GLuint shadowShaderProgram, const glm::vec3& shadowCenter, float halfExtent);
 
     /**
      * Render smoke particles (call after render())