Not THAT Slow - Update Movespeed Modifiers for Smooth Movement (#22127)

The /tg/-style movespeed_modifier datums were not incorporated into the
original smooth movement PR. While
https://github.com/Aurorastation/Aurora.3/pull/22043 restores that
functionality, the original values no longer feel good now that we're on
smooth movement. This PR halves ALL movespeed_modifier/slowdown datums,
with the exception of species-native, as a baseline before more
individual adjustments are made.

code/modules/movespeed/modifiers/mobs.dm

@@ -7,7 +7,7 @@
 /datum/movespeed_modifier/equipment_speedmod/immutable
 
 /datum/movespeed_modifier/config_walk_run
-	multiplicative_slowdown = 1
+	multiplicative_slowdown = 0.5
 	id = MOVESPEED_ID_MOB_WALK_RUN
 	flags = IGNORE_NOSLOW
 
@@ -26,7 +26,7 @@
 	flags = IGNORE_NOSLOW
 
 /datum/movespeed_modifier/encumbered
-	multiplicative_slowdown = 2
+	multiplicative_slowdown = 1
 	id = "encumbered"
 
 /datum/movespeed_modifier/zoomzoomkitty

code/modules/movespeed/modifiers/reagents.dm

@@ -1,43 +1,69 @@
 ABSTRACT_TYPE(/datum/movespeed_modifier/reagent)
 	blacklisted_movetypes = (FLYING|FLOATING)
 
+// These movespeed_modifiers are derived from metabolizing reagents, and if stacked can cause characters to move faster than ghosts
+// For all reagents which increase movespeed, we give them the id 'reagent_speedup' and a priority correlating with their mutliplicative slowdown.
+// This ensures that if a player ingests multiple reagents which increase movement speed, only the greatest movespeed bonus will apply.
+// While we could do some fancy shit with weighting for multiple speedup reagents, it is best to keep this behavior simple for gameplay.
+
 //Depends on the amount of caffeine in the drink
 /datum/movespeed_modifier/reagent/caffeine
+	id = "reagent_speedup"
 	variable = TRUE
 
 /datum/movespeed_modifier/reagent/hyperzine
-	multiplicative_slowdown = -1
-
-/datum/movespeed_modifier/reagent/stimm
-	multiplicative_slowdown = -1.5
-
-/datum/movespeed_modifier/reagent/skrell_nootropic
+	id = "reagent_speedup"
+	priority = 50
 	multiplicative_slowdown = -0.5
 
+/datum/movespeed_modifier/reagent/stimm
+	id = "reagent_speedup"
+	priority = 75
+	multiplicative_slowdown = -0.75
+
+/datum/movespeed_modifier/reagent/skrell_nootropic
+	id = "reagent_speedup"
+	priority = 25
+	multiplicative_slowdown = -0.25
+
 /datum/movespeed_modifier/reagent/dionae_stimulant
-	multiplicative_slowdown = -1
+	id = "reagent_speedup"
+	priority = 50
+	multiplicative_slowdown = -0.5
 
 /datum/movespeed_modifier/reagent/kokoreed
-	multiplicative_slowdown = -1
+	id = "reagent_speedup"
+	priority = 50
+	multiplicative_slowdown = -0.5
 
 /datum/movespeed_modifier/reagent/kilosemine
-	multiplicative_slowdown = -1
+	id = "reagent_speedup"
+	priority = 50
+	multiplicative_slowdown = -0.5
 
 ABSTRACT_TYPE(/datum/movespeed_modifier/reagent/zorasoda)
 
 /datum/movespeed_modifier/reagent/zorasoda/drone
-	multiplicative_slowdown = -1
+	id = "reagent_speedup"
+	priority = 50
+	multiplicative_slowdown = -0.5
 
 
 ABSTRACT_TYPE(/datum/movespeed_modifier/alcohol)
 
+
 ///This is ONLY for the intoxication management of alcohol, NOT for anything else
 ///if you're not using it in `code\modules\mob\living\carbon\human\intoxication.dm`, you're doing it wrong
+/// As of the introduction of removing stacking reagent movespeed modifiers, both alcohol AND any non-alcohol 'slowdown' chems should use the same id
+/// and priority system as used in 'speedup' chems.
 /datum/movespeed_modifier/alcohol/intoxication
-	multiplicative_slowdown = 2
+	id = "reagent_slowdown"
+	priority = 50
+	multiplicative_slowdown = 0.3
 	variable = TRUE
 
 //Only speed boost for unathi
 /datum/movespeed_modifier/alcohol/butanol
+	id = "reagent_slowdown"
 	multiplicative_slowdown = 0
 	variable = TRUE

code/modules/movespeed/modifiers/spells.dm

@@ -3,4 +3,4 @@ ABSTRACT_TYPE(/datum/movespeed_modifier/spell)
 ABSTRACT_TYPE(/datum/movespeed_modifier/spell/technomancer)
 
 /datum/movespeed_modifier/spell/technomancer/haste
-	multiplicative_slowdown = -0.8
+	multiplicative_slowdown = -0.4

code/modules/movespeed/modifiers/synthetics.dm

@@ -1,9 +1,9 @@
 /datum/movespeed_modifier/synth_emp
 	id = "synthemp"
 	variable = TRUE
-	multiplicative_slowdown = 1
+	multiplicative_slowdown = 0.5
 
 /datum/movespeed_modifier/endoskeleton
 	id = "synthendoskeleton"
 	variable = TRUE
-	multiplicative_slowdown = 1
+	multiplicative_slowdown = 0.5