Puts blood back on floor plane, puts runes on floor plane (removes shadows from blood and runes) (#87013)

## About The Pull Request

Basically goes through and puts some flat objects onto the floor plane.
Also puts blood back on the floor plane, since it seems like it used to
be but may have been moved mistakenly questionmark.

This means they will not be affected by Ambient Occlusion: 


![image](https://github.com/user-attachments/assets/594b878b-e5e9-42e3-900e-3c9390c819ee)

## Why It's Good For The Game

A lot of these "flat" things look really out of place because they're
secretly "floating" due to being on the wrong plane

## Changelog

🆑 Melbert
qol: Runes, crayons, and similar decals no longer have shadows
fix: Blood and similar "mess" decals no longer have shadows (again)
/🆑

code/__DEFINES/layers.dm

@@ -151,6 +151,11 @@
 #define ABOVE_OPEN_TURF_LAYER (12 + TOPDOWN_LAYER)
 ///catwalk overlay of /turf/open/floor/plating/catwalk_floor
 #define CATWALK_LAYER (13 + TOPDOWN_LAYER)
+#define LOWER_RUNE_LAYER (14 + TOPDOWN_LAYER)
+#define RUNE_LAYER (15 + TOPDOWN_LAYER)
+/// [GAME_CLEAN_LAYER] but for floors.
+/// Basically any layer below this (numerically) is "on" a floor for the purposes of washing
+#define FLOOR_CLEAN_LAYER (20 + TOPDOWN_LAYER)
 
 //WALL_PLANE layers
 #define BELOW_CLOSED_TURF_LAYER 2.053
@@ -169,12 +174,10 @@
 #define PLUMBING_PIPE_VISIBILE_LAYER 2.495//layer = initial(layer) + ducting_layer / 3333 in atmospherics/handle_layer() to determine order of duct overlap
 #define BOT_PATH_LAYER 2.497
 #define LOW_OBJ_LAYER 2.5
-#define LOW_SIGIL_LAYER 2.52
-#define SIGIL_LAYER 2.53
 #define HIGH_PIPE_LAYER 2.54
 // Anything above this layer is not "on" a turf for the purposes of washing
 // I hate this life of ours
-#define FLOOR_CLEAN_LAYER 2.55
+#define GAME_CLEAN_LAYER 2.55
 #define TRAM_STRUCTURE_LAYER 2.57
 #define TRAM_FLOOR_LAYER 2.58
 #define TRAM_WALL_LAYER 2.59