Implement Godot grid viewport rendering

TASKS.md

@@ -45,7 +45,7 @@ The Godot frontend (src/ReactorMaintenance.Godot) has a complete UX scaffold (sp
 
 **Goal:** Replace the grid placeholder with a functional, interactive tile-based viewport.
 
-- [ ] **Task 2.1: Create GridViewport control**
+- [x] **Task 2.1: Create GridViewport control**
   - Location: src/ReactorMaintenance.Godot/Controls/GridViewport.cs
   - Custom Control that renders the level grid via CanvasItem._draw()
   - Supports configurable tile size (default 48px)
@@ -53,35 +53,35 @@ The Godot frontend (src/ReactorMaintenance.Godot) has a complete UX scaffold (sp
   - Exposes SelectedCell, RobotPosition, HoveredCell properties
   - Fires OnCellSelected, OnCellHovered, OnGridClicked signals
 
-- [ ] **Task 2.2: Implement terrain rendering**
+- [x] **Task 2.2: Implement terrain rendering**
   - Draw floor tiles with graphite/steel palette per ART.md
   - Draw wall tiles with darker steel, reinforced silhouettes
   - Apply subtle hue shifts between adjacent floor plates (no checkerboard)
   - Support layer opacity rules (surface full / underground 25% or 50%)
 
-- [ ] **Task 2.3: Implement prop rendering**
+- [x] **Task 2.3: Implement prop rendering**
   - Draw props at cell centers with semantic colors
   - Show state indicators (enabled/disabled, producing/starved)
   - Use generated asset textures where available, fall back to procedural badges
   - Support prop highlight for selected cell
 
-- [ ] **Task 2.4: Implement robot marker**
+- [x] **Task 2.4: Implement robot marker**
   - Draw robot sprite at current RobotPosition
   - Show direction indicator
   - Use maintenance_robot.png from Assets/Characters
   - Animate on position change (optional, low priority)
 
-- [ ] **Task 2.5: Implement hazard rendering**
+- [x] **Task 2.5: Implement hazard rendering**
   - Draw surface hazards on affected floor cells
   - Use semantic colors: fuel (red slicks), coolant (blue-cyan), electricity (yellow arcs), heat (orange-white)
   - Show hazard intensity via saturation/opacity
 
-- [ ] **Task 2.6: Implement cell selection highlight**
+- [x] **Task 2.6: Implement cell selection highlight**
   - Draw selection rectangle around selected cell
   - Draw reachable/actionable hints on valid move targets
   - Support hover highlight on non-selected cells
 
-- [ ] **Task 2.7: Implement underground network visualization**
+- [x] **Task 2.7: Implement underground network visualization**
   - Draw carrier-colored network lines between adjacent cells with positive flow
   - Fuel = red, Coolant = blue, Electricity = yellow
   - Render sources as large centered dots