✨ fix depth sorting for world objects by tileY

Fixes #31. All trees, rocks, seedlings and buildings now use
tileY+5 as depth instead of a fixed value, so objects further
down the screen always render in front of objects above them
regardless of spawn order. Build ghost moved to depth 1000/1001.

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

src/systems/BuildingSystem.ts

@@ -32,7 +32,7 @@ export class BuildingSystem {
 
   create(): void {
     this.ghost = this.scene.add.rectangle(0, 0, TILE_SIZE, TILE_SIZE, 0x00FF00, 0.35)
-    this.ghost.setDepth(20)
+    this.ghost.setDepth(1000)
     this.ghost.setVisible(false)
     this.ghost.setStrokeStyle(2, 0x00FF00, 0.8)
 
@@ -40,7 +40,7 @@ export class BuildingSystem {
       fontSize: '10px', color: '#ffffff', fontFamily: 'monospace',
       backgroundColor: '#000000aa', padding: { x: 3, y: 2 }
     })
-    this.ghostLabel.setDepth(21)
+    this.ghostLabel.setDepth(1001)
     this.ghostLabel.setVisible(false)
     this.ghostLabel.setOrigin(0.5, 1)