Add editor image badges and drag moves

docs/design.md

@@ -455,6 +455,8 @@ The editor includes layer selection for Surface, Electricity, Fuel, and Coolant:
 - Networks render as thick lines connecting adjacent cell centers; sources render as large centered dots.
 - Tools are layer-aware. Cursor is always available. Surface terrain, props, consumers, hazards, doors, and heat tools are available only on Surface. Network painting and sources are available only on their matching underground layer.
 
+Editor tool badges and drag previews use stable semantic image keys when assets are available. Assets may be added under `Images/Badges` or `Images/Elements` with filenames such as `tool-door.png`, `prop-reactor.png`, `carrier-fuel-source.png`, `leak-electricity.png`, or `robot.png`; missing assets fall back to compact procedural badges and text labels.
+
 The serialized level schema stores level metadata, dimensions, terrain, underground layers including structural integrity, props and prop state, required reactor consumer counts, leaks, robot state, inventory, forecasts, and dynamic state when saving active play.
 
 The loader accepts only schema-valid level data and returns clear errors for malformed data.