Fix grid layers with z_index=0 rendering on top of entities, closes #257

Layers with z_index <= 0 now render below entities (ground level), and
only layers with z_index > 0 render above entities. Previously z_index=0
was treated as "above entities" which was unintuitive -- entities stand
on ground level (z=0) with their feet, so z=0 layers should be beneath.

Changed in both UIGrid.cpp and UIGridView.cpp render methods:
- "z_index >= 0" to "z_index > 0" for break condition
- "z_index < 0" to "z_index <= 0" for skip condition

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/GridLayers.h

@@ -30,7 +30,7 @@ public:
 
     GridLayerType type;
     std::string name;      // #150 - Layer name for GridPoint property access
-    int z_index;           // Negative = below entities, >= 0 = above entities
+    int z_index;           // <= 0 = below entities (ground level), > 0 = above entities
     int grid_x, grid_y;    // Dimensions
     GridData* parent_grid;   // Parent grid reference (#252: GridData, not UIGrid)
     bool visible;          // Visibility flag

src/UIGrid.cpp

@@ -182,10 +182,10 @@ void UIGrid::render(sf::Vector2f offset, sf::RenderTarget& target)
     if (y_limit > grid_h) y_limit = grid_h;
 
     // #150 - Layers are now the sole source of grid rendering (base layer removed)
-    // Render layers with z_index < 0 (below entities)
+    // Render layers with z_index <= 0 (below entities)
     sortLayers();
     for (auto& layer : layers) {
-        if (layer->z_index >= 0) break;  // Stop at layers that go above entities
+        if (layer->z_index > 0) break;  // Stop at layers that go above entities (#257)
         layer->render(*activeTexture, left_spritepixels, top_spritepixels,
                      left_edge, top_edge, x_limit, y_limit, zoom, cell_width, cell_height);
     }
@@ -222,9 +222,9 @@ void UIGrid::render(sf::Vector2f offset, sf::RenderTarget& target)
         Resources::game->metrics.totalEntities += totalEntities;
     }
 
-    // #147 - Render dynamic layers with z_index >= 0 (above entities)
+    // #147 - Render dynamic layers with z_index > 0 (above entities)
     for (auto& layer : layers) {
-        if (layer->z_index < 0) continue;  // Skip layers below entities
+        if (layer->z_index <= 0) continue;  // Skip layers at or below entities (#257)
         layer->render(*activeTexture, left_spritepixels, top_spritepixels,
                      left_edge, top_edge, x_limit, y_limit, zoom, cell_width, cell_height);
     }

src/UIGridView.cpp

@@ -167,10 +167,10 @@ void UIGridView::render(sf::Vector2f offset, sf::RenderTarget& target)
     int y_limit = top_edge + height_sq + 2;
     if (y_limit > grid_data->grid_h) y_limit = grid_data->grid_h;
 
-    // Render layers below entities (z_index < 0)
+    // Render layers below entities (z_index <= 0)
     grid_data->sortLayers();
     for (auto& layer : grid_data->layers) {
-        if (layer->z_index >= 0) break;
+        if (layer->z_index > 0) break;  // #257: z_index=0 is ground level (below entities)
         layer->render(*activeTexture, left_spritepixels, top_spritepixels,
                      left_edge, top_edge, x_limit, y_limit, zoom, cell_width, cell_height);
     }
@@ -191,9 +191,9 @@ void UIGridView::render(sf::Vector2f offset, sf::RenderTarget& target)
         }
     }
 
-    // Render layers above entities (z_index >= 0)
+    // Render layers above entities (z_index > 0)
     for (auto& layer : grid_data->layers) {
-        if (layer->z_index < 0) continue;
+        if (layer->z_index <= 0) continue;  // #257: skip ground-level and below
         layer->render(*activeTexture, left_spritepixels, top_spritepixels,
                      left_edge, top_edge, x_limit, y_limit, zoom, cell_width, cell_height);
     }