Faint block highlight on hover and inside the dig rectangle

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

DESIGN.md

@@ -19,6 +19,7 @@ An original dungeon management game (genre: dungeon building & defence) that run
 - Units: **pixel sprites as billboards** (always facing the camera)
 - Lighting: **dark + torches** – dark base level; claimed areas and light sources (torches, core, portal, gate) are bright, flickering light
 - **Dig marking**: a marked block glows as a translucent yellow box with a yellow outline around all edges
+- **Hover**: the wall block under the mouse gets a faint version of that box (in the colour of the active tool); while dragging a dig rectangle, every block inside is tinted faintly; floor tiles get an outline
 - **Tile outlines**: subtle dark lines around every tile (floors, wall tops, vertical wall edges), following the irregular surface; toggle with G (later in the graphics settings)
 - The UI is a 2D layer on top, including a minimap