Tweaks turf add overlay logic (#23071)

## About PR

Makes it so when a turf calls the overlay subsystem (if overlay layer
was left as default) it'll add overlays slightly above the turf layer.
With this we'll no longer have the silly issue where turf smoothing
overlays appear under the adjacent turfs depending on turf load order

In short, here's what it fixes

Before:

<img width="396" height="601" alt="Screenshot_10"
src="https://github.com/user-attachments/assets/91708402-c6c6-433f-af9f-5293ce926731"
/>

After:

<img width="391" height="598" alt="Screenshot_9"
src="https://github.com/user-attachments/assets/556d48dd-d463-4f76-b075-f0d77340e837"
/>

---

to-do:
- [x] Figure out whether to change `is_plating()` check method or add
the missing `initial_flooring` for every exoplanet turf type since this
plays a role in deciding decal layer, suffer as both paths are equally
agonizing

code/modules/mining/mine_turfs.dm

@@ -768,8 +768,10 @@ GLOBAL_LIST_INIT(mineral_can_smooth_with, list(
 
 GLOBAL_LIST_INIT(asteroid_floor_smooth, list(
 	/turf/simulated/floor/exoplanet/asteroid/ash,
+	/turf/simulated/wall,
+	/turf/unsimulated/wall,
 	/turf/simulated/mineral,
-	/turf/simulated/wall
+	/turf/unsimulated/mineral
 ))
 
 // Copypaste parent for performance.