Stack overflow detection for the Master Controller. (#56008)

* Stack overflow detection for the Master Controller.

Using a weakref, we can detect if the mc's stack was ended by byond due to a stack overflow, and restart it without waiting the entire defcon countdown in the failsafe controller.

I built a system around this concept under /datum/stack_end_detector and deployed it to the MC's main loop with checks in the failsafe controller.

code/datums/helper_datums/stack_end_detector.dm

@@ -0,0 +1,32 @@
+/**
+	Stack End Detector.
+	Can detect if a given code stack has exited, used by the mc for stack overflow detection.
+
+ **/
+/datum/stack_end_detector
+	var/datum/weakref/_WF
+	var/datum/stack_canary/_canary
+
+/datum/stack_end_detector/New()
+	_canary = new()
+	_WF = WEAKREF(_canary)
+
+/** Prime the stack overflow detector.
+	Store the return value of this proc call in a proc level var.
+	Can only be called once.
+**/
+/datum/stack_end_detector/proc/prime_canary()
+	if (!_canary)
+		CRASH("Prime_canary called twice")
+	. = _canary
+	_canary = null
+
+/// Returns true if the stack is still going. Calling before the canary has been primed also returns true
+/datum/stack_end_detector/proc/check()
+	return !!_WF.resolve()
+
+/// Stack canary. Will go away if the stack it was primed by is ended by byond for return or stack overflow reasons.
+/datum/stack_canary
+
+/// empty proc to avoid warnings about unused variables. Call this proc on your canary in the stack it's watching.
+/datum/stack_canary/proc/use_variable()