✨ 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/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)
   }