Kill Gas Tank Processing (#22810)

This PR nukes the single largest and most common source of unnecessary
Process() calls, the Air Tanks. Air tanks now dynamically add and remove
themselves from processing only when actually required (such as by being
actively used) or they contain a "reactive" gas mixture.

Gas tanks made up the overwhelming majority of all process calls, and
without them always being on, the Processing subsystem becomes extremely
cheap.

<img width="394" height="33" alt="image"
src="https://github.com/user-attachments/assets/a973b863-56ad-46f6-ac34-e3cd479dd76d"
/>

I have also tested gas tanks to make sure that they still work, being
chargable, dischargable, able to breathe from them, and that opening the
valve works. All 4 actions add the gas tank to processing. When the
actions are finished, the tank exits processing.

<img width="686" height="388" alt="image"
src="https://github.com/user-attachments/assets/e91c96d3-9031-4720-8e21-aa4d4afa84fc"
/>

code/game/objects/items/weapons/tanks/tanks.dm

@@ -183,6 +183,7 @@
 		. = TRUE
 
 /obj/item/tank/remove_air(amount)
+	START_PROCESSING(SSprocessing, src)
 	return air_contents.remove(amount)
 
 /obj/item/tank/return_air()
@@ -190,14 +191,20 @@
 
 /obj/item/tank/assume_air(datum/gas_mixture/giver)
 	air_contents.merge(giver)
-
+	START_PROCESSING(SSprocessing, src)
 	check_status()
+	update_gauge()
 	return 1
 
+/obj/item/tank/update_icon()
+	. = ..()
+	update_gauge()
+
 /obj/item/tank/proc/remove_air_volume(volume_to_return)
 	if(!air_contents)
 		return null
 
+	START_PROCESSING(SSprocessing, src)
 	var/tank_pressure = XGM_PRESSURE(air_contents)
 	if(tank_pressure < distribute_pressure)
 		distribute_pressure = tank_pressure
@@ -210,17 +217,29 @@
 	var/tank_pressure = 0
 	// we pass tank_pressure around and try not to recalc it unless we have to
 	// this is a very hot proc (~2M calls/hr)
-	if(air_contents)
-		tank_pressure = XGM_PRESSURE(air_contents)
-		air_contents.react()
-		tank_pressure = check_status(tank_pressure)
-	if(gauge_icon)
-		update_gauge(tank_pressure)
+	if(!air_contents)
+		update_gauge(tank_pressure) // Just to make sure the tank is marked empty
+		return PROCESS_KILL // No need to process empty air tanks
+
+	tank_pressure = XGM_PRESSURE(air_contents)
+	if (!air_contents.react())
+		update_gauge(tank_pressure) // Last icon cleanup on no-reaction.
+		return PROCESS_KILL // No need to continuously process non-reactive gas mixtures.
+
+	tank_pressure = check_status(tank_pressure)
+	if (!tank_pressure)
+		update_gauge(tank_pressure) // Last icon cleanup on reaction consuming all the gas
+		return PROCESS_KILL // No need to continuously process empty gas mixtures.
+
+	update_gauge(tank_pressure)
 
 /obj/item/tank/proc/adjust_initial_gas()
 	return
 
 /obj/item/tank/proc/update_gauge(gauge_pressure = 0)
+	if (!gauge_icon)
+		return
+
 	if(air_contents)
 		if(!gauge_pressure)
 			gauge_pressure = XGM_PRESSURE(air_contents)
@@ -307,5 +326,6 @@
 	return QDELETED(src) ? 0 : pressure // if we qdel'd or return 0, something changed and we gotta recalc
 
 /obj/item/tank/proc/remove_air_by_flag(flag, amount)
+	START_PROCESSING(SSprocessing, src)
 	. = air_contents.remove_by_flag(flag, amount)
 	update_icon()

code/game/objects/structures/machinery/atmoalter/portable_atmospherics.dm

@@ -105,6 +105,15 @@
 	if (network)
 		network.update = 1
 
+// This exists because for stupid reasons these machines touch air directly instead of respecting an air tank's procs.
+/obj/structure/machinery/portable_atmospherics/Exited(atom/movable/gone, direction)
+	. = ..()
+	if(gone == holding)
+		if(istype(gone, /obj/item/tank))
+			var/obj/item/tank/T = gone
+			T.update_icon()
+		holding = null
+
 /obj/structure/machinery/portable_atmospherics/attackby(obj/item/attacking_item, mob/user)
 	if ((istype(attacking_item, /obj/item/tank) && !( src.destroyed )))
 		if (src.holding)