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/controllers/subsystems/overlays.dm

@@ -98,25 +98,40 @@ SUBSYSTEM_DEF(overlays)
 	var/list/result = list()
 	var/icon/icon = subject.icon
 	var/atom/entry
+	var/overlay_appearance
 	for (var/i = 1 to length(sources))
 		entry = sources[i]
 		if (!entry)
 			continue
 		else if (istext(entry))
-			result += GetStateAppearance(icon, entry)
+			overlay_appearance = GetStateAppearance(icon, entry)
 		else if (isicon(entry))
-			result += GetIconAppearance(entry)
+			overlay_appearance = GetIconAppearance(entry)
 		else if (istype(entry, /mutable_appearance))
-			result += entry
+			overlay_appearance = entry
 		else
 			if (isloc(entry))
 				if (entry.atom_flags & ATOM_AWAITING_OVERLAY_UPDATE)
 					entry.UpdateOverlays()
 			if (!ispath(entry))
-				result += entry.appearance
+				overlay_appearance = entry.appearance
 			else
 				var/image/image = entry
-				result += image.appearance
+				overlay_appearance = image.appearance
+
+		// If the caller is a turf, we put the overlay's layer slightly above it.
+		// So this way overlays extending over to neighbouring turfs don't appear under them depending on load order.
+		// We only apply this if source hasn't explictly provided a layer (think the FLOAT_LAYER as a default layer for overlays).
+		// For more info see overlays doc. in byond ref.
+		var/turf/T = subject
+		if(istype(T))
+			var/mutable_appearance/turf_overlay = new()
+			turf_overlay.appearance = overlay_appearance
+			if(turf_overlay.layer == FLOAT_LAYER)
+				turf_overlay.layer = T.is_plating() ? ABOVE_PLATING_LAYER : TURF_DETAIL_LAYER
+				overlay_appearance = turf_overlay.appearance
+
+		result += overlay_appearance
 	return result
 
 
@@ -306,4 +321,3 @@ SUBSYSTEM_DEF(overlays)
 		overlays |= other.atom_protected_overlay_cache
 	if (cache_target & ATOM_ICON_CACHE_NORMAL)
 		overlays |= other.atom_overlay_cache
-