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/terrain_renderer.cpp

@@ -369,7 +369,7 @@ GLuint TerrainRenderer::createAlphaTexture(const std::vector<uint8_t>& alphaData
     return textureID;
 }
 
-void TerrainRenderer::renderShadow(GLuint shaderProgram) {
+void TerrainRenderer::renderShadow(GLuint shaderProgram, const glm::vec3& shadowCenter, float halfExtent) {
     if (chunks.empty()) return;
 
     GLint modelLoc = glGetUniformLocation(shaderProgram, "uModel");
@@ -378,6 +378,13 @@ void TerrainRenderer::renderShadow(GLuint shaderProgram) {
 
     for (const auto& chunk : chunks) {
         if (!chunk.isValid()) continue;
+
+        // Cull chunks whose bounding sphere doesn't overlap the shadow frustum (XY plane)
+        float maxDist = halfExtent + chunk.boundingSphereRadius;
+        float dx = chunk.boundingSphereCenter.x - shadowCenter.x;
+        float dy = chunk.boundingSphereCenter.y - shadowCenter.y;
+        if (dx * dx + dy * dy > maxDist * maxDist) continue;
+
         glBindVertexArray(chunk.vao);
         glDrawElements(GL_TRIANGLES, chunk.indexCount, GL_UNSIGNED_INT, 0);
         glBindVertexArray(0);