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.

src/rendering/m2_renderer.cpp

@@ -2151,7 +2151,7 @@ void M2Renderer::render(const Camera& camera, const glm::mat4& view, const glm::
     }
 }
 
-void M2Renderer::renderShadow(GLuint shadowShaderProgram) {
+void M2Renderer::renderShadow(GLuint shadowShaderProgram, const glm::vec3& shadowCenter, float halfExtent) {
     if (instances.empty() || shadowShaderProgram == 0) {
         return;
     }
@@ -2172,6 +2172,12 @@ void M2Renderer::renderShadow(GLuint shadowShaderProgram) {
     glActiveTexture(GL_TEXTURE0);
 
     for (const auto& instance : instances) {
+        // Cull instances whose AABB doesn't overlap the shadow frustum (XY plane)
+        glm::vec3 instCenter = (instance.worldBoundsMin + instance.worldBoundsMax) * 0.5f;
+        glm::vec3 instHalf   = (instance.worldBoundsMax - instance.worldBoundsMin) * 0.5f;
+        if (std::abs(instCenter.x - shadowCenter.x) > halfExtent + instHalf.x) continue;
+        if (std::abs(instCenter.y - shadowCenter.y) > halfExtent + instHalf.y) continue;
+
         auto it = models.find(instance.modelId);
         if (it == models.end()) continue;