✨ 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)
 

src/systems/ResourceSystem.ts

@@ -47,10 +47,10 @@ export class ResourceSystem {
       sprite.setOrigin(0.5, 0.75)
     }
 
-    sprite.setDepth(5)
+    sprite.setDepth(node.tileY + 5)
 
     const healthBar = this.scene.add.graphics()
-    healthBar.setDepth(6)
+    healthBar.setDepth(node.tileY + 6)
     healthBar.setVisible(false)
 
     this.sprites.set(node.id, { sprite, node, healthBar })

src/systems/TreeSeedlingSystem.ts

@@ -110,7 +110,7 @@ export class TreeSeedlingSystem {
     const key = `seedling_${Math.min(s.stage, 2)}`
     const sprite = this.scene.add.image(x, y, key)
       .setOrigin(0.5, 0.85)
-      .setDepth(5)
+      .setDepth(s.tileY + 5)
     this.sprites.set(s.id, sprite)
   }