snails (#90505)

## About The Pull Request
adds snails to the game

![image](https://github.com/user-attachments/assets/f1745795-695f-443a-827e-1f1987d4fdb6)

these are harmless critters you can find in maints. they love eating all
variety of fruits, and are gravitated towards snail people, where
they'll dance around them.

you can also pick them up and put them on your head.

finally, you can also grab them and put them in hydrotrays (they will
swim around in it very slowly). they'll help ur plants grow as they act
as natural weed-repellants, as they'll eat weeds that grow in trays.

![image](https://github.com/user-attachments/assets/512f0974-d0d1-4f90-a540-c03d1c969328)


## Why It's Good For The Game
there's not that many mobs you can usually find in maints, currently
there's only mice and cockroaches, this helps expand the pools a bit.

## Changelog
🆑
add: adds snails to the game. (keep them away from salt!)
/🆑

code/controllers/subsystem/minor_mapping.dm

@@ -1,4 +1,3 @@
-#define PROB_MOUSE_SPAWN 98
 #define PROB_SPIDER_REPLACEMENT 50
 
 SUBSYSTEM_DEF(minor_mapping)
@@ -8,6 +7,12 @@ SUBSYSTEM_DEF(minor_mapping)
 		/datum/controller/subsystem/atoms,
 	)
 	flags = SS_NO_FIRE
+	///a list of vermin we pick from to spawn.
+	var/list/vermin_chances = list(
+		/mob/living/basic/mouse = 80,
+		/mob/living/basic/snail = 18,
+		/mob/living/basic/regal_rat/controlled = 2,
+	)
 
 /datum/controller/subsystem/minor_mapping/Initialize()
 // This whole subsystem just introduces a lot of odd confounding variables into unit test situations,
@@ -30,14 +35,14 @@ SUBSYSTEM_DEF(minor_mapping)
 			continue
 
 		to_spawn--
-		if(HAS_TRAIT(SSstation, STATION_TRAIT_SPIDER_INFESTATION) && prob(PROB_SPIDER_REPLACEMENT))
-			new /mob/living/basic/spider/maintenance(proposed_turf)
-			return
 
-		if (prob(PROB_MOUSE_SPAWN))
-			new /mob/living/basic/mouse(proposed_turf)
+		var/picked_path
+		if(HAS_TRAIT(SSstation, STATION_TRAIT_SPIDER_INFESTATION) && prob(PROB_SPIDER_REPLACEMENT))
+			picked_path = /mob/living/basic/spider/maintenance
 		else
-			new /mob/living/basic/regal_rat/controlled(proposed_turf)
+			picked_path = pick_weight(vermin_chances)
+
+		new picked_path(proposed_turf)
 
 /// Returns true if a mouse won't die if spawned on this turf
 /datum/controller/subsystem/minor_mapping/proc/valid_mouse_turf(turf/open/proposed_turf)
@@ -91,5 +96,4 @@ SUBSYSTEM_DEF(minor_mapping)
 
 	return shuffle(suitable)
 
-#undef PROB_MOUSE_SPAWN
 #undef PROB_SPIDER_REPLACEMENT