Phase 28 Session 3: Debug borders for chunk visualization

Added visual debugging to verify chunk rendering:

Flat2DTerrainSystem.js changes:
- Added red border around each rendered chunk
- Border: 2px red, semi-transparent (0.5 alpha)
- High depth (100) to ensure visibility
- Console log for each border created

Purpose:
- Verify chunks are actually being rendered
- See chunk boundaries visually
- Debug if chunks overlap or have gaps
- Confirm 3x3 chunk grid (9 chunks = 9 red boxes)

Expected visual:
- 9 red rectangles around player (3x3 grid)
- Each rectangle = 50x50 tiles = 2400x2400 pixels
- Player should be in center rectangle

Session 3: Visual testing in progress
Next: Reload and verify red borders appear

src/systems/Flat2DTerrainSystem.js

@@ -396,6 +396,29 @@ class Flat2DTerrainSystem {
         }
 
         console.log(`✅ Chunk (${chunk.chunkX}, ${chunk.chunkY}) rendered with biomes`);
+
+        // 🐛 DEBUG: Add visible border around chunk
+        if (this.scene.add) {
+            const chunkWorldX = chunk.chunkX * (this.chunkSize * size);
+            const chunkWorldY = chunk.chunkY * (this.chunkSize * size);
+            const chunkPixelSize = this.chunkSize * size;
+
+            const border = this.scene.add.rectangle(
+                chunkWorldX,
+                chunkWorldY,
+                chunkPixelSize,
+                chunkPixelSize
+            );
+            border.setOrigin(0, 0);
+            border.setStrokeStyle(2, 0xFF0000, 0.5); // Red semi-transparent border
+            border.setDepth(100); // High depth to see it
+            border.setFillStyle(0x000000, 0); // No fill
+
+            if (!chunk.sprites) chunk.sprites = [];
+            chunk.sprites.push(border);
+
+            console.log(`🔲 Debug border added at (${chunkWorldX}, ${chunkWorldY})`);
+        }
     }
 
     // Add biome-specific feature